Unified cooling for multiple polyolefin polymerization reactors

ABSTRACT

A system and method for a polyolefin reactor temperature control system having a first reactor temperature control path, a second reactor temperature control path, and a shared temperature control path. The shared temperature control path is configured to combine and process coolant return streams, and to provide coolant supply for the first reactor temperature control path and the second reactor temperature control path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/429,692, filed Feb. 10, 2017, which is a continuation of U.S. patentapplication Ser. No. 15/086,598, filed Mar. 31, 2016, now U.S. Pat. No.9,624,316, which is a continuation of U.S. patent application Ser. No.13/872,746, filed Apr. 29, 2013, now U.S. Pat. No. 9,310,137, each ofwhich is incorporated herein by reference in their entireties.

BACKGROUND Field of the Invention

The present invention relates generally to polyolefin production withmultiple polymerization reactors and, more particularly, to employingindividual reactor temperature control having shared cooling capacityfor the multiple polymerization reactors.

Description of the Related Art

This section is intended to introduce the reader to aspects of art thatmay be related to aspects of the present invention, which are describedand/or claimed below. This discussion is believed to be helpful inproviding the reader with background information to facilitate a betterunderstanding of the various aspects of the present invention.Accordingly, it should be understood that these statements are to beread in this light, and not as admissions of prior art.

As chemical and petrochemical technologies have advanced, the productsof these technologies have become increasingly prevalent in society. Inparticular, as techniques for bonding simple molecular building blocksinto longer chains (or polymers) have advanced, the polymer products,typically in the form of various plastics, have been increasinglyincorporated into everyday items. Polyolefin polymers such aspolyethylene, polypropylene, and their copolymers, are used for piping,retail and pharmaceutical packaging, food and beverage packaging,plastic bags, toys, carpeting, various industrial products, automobilecomponents, appliances and other household items, and so forth.

Specific types of polyolefins, such as high-density polyethylene (HDPE),have particular applications in the manufacture of blow-molded andinjection-molded goods, such as food and beverage containers, film, andplastic pipe. Other types of polyolefins, such as low-densitypolyethylene (LDPE), linear low-density polyethylene (LLDPE), isotacticpolypropylene (iPP), and syndiotactic polypropylene (sPP) are alsosuited for similar applications. The mechanical requirements of theapplication, such as tensile strength and density, and/or the chemicalrequirements, such thermal stability, molecular weight, and chemicalreactivity, typically determine what type of polyolefin is suitable.

One benefit of polyolefin construction, as may be deduced from the listof uses above, is that it is generally non-reactive with goods orproducts with which it is in contact. This allows polyolefin products tobe used in residential, commercial, and industrial contexts, includingfood and beverage storage and transportation, consumer electronics,agriculture, shipping, and vehicular construction. The wide variety ofresidential, commercial and industrial uses for polyolefins hastranslated into a substantial demand for raw polyolefin which can beextruded, injected, blown or otherwise formed into a final consumableproduct or component.

To satisfy this demand, various processes exist by which olefins may bepolymerized to form polyolefins. These processes may be performed at ornear petrochemical facilities, which provide ready access to theshort-chain olefin molecules (monomers and comonomers), such asethylene, propylene, butene, pentene, hexene, octene, decene, and otherbuilding blocks of the much longer polyolefin polymers. These monomersand comonomers may be polymerized in a liquid-phase polymerizationreactor and/or gas-phase polymerization reactor. As polymer chainsdevelop during polymerization in the reactor, solid particles known as“fluff” or “flake” or “powder” are produced in the reactor.

The fluff may possess one or more melt, physical, rheological, and/ormechanical properties of interest, such as density, melt index (MI),melt flow rate (MFR), comonomer content, molecular weight,crystallinity, and so on. Different properties for the fluff may bedesirable depending on the application to which the polyolefin fluff orsubsequently pelletized polylefin is to be applied. Selection andcontrol of the reaction conditions within the reactor, such astemperature, pressure, chemical concentrations, polymer production rate,catalyst type, and so forth, may affect the fluff properties.

In addition to the one or more olefin monomers, a catalyst (e.g.,Ziegler-Natta, metallocene, chromium-based, post-metallocene, nickel,etc.) for facilitating the polymerization of the monomers may be addedto the reactor. For example, the catalyst may be a particle added via areactor feed stream and, once added, suspended in the fluid mediumwithin the reactor. Unlike the monomers, catalysts are generally notconsumed in the polymerization reaction. Moreover, an inert hydrocarbon,such as isobutane, propane, n-pentane, i-pentane, neopentane, n-hexane,and/or heptane, and so on, may be added to the reactor and utilized as adiluent to carry the contents of the reactor. However, somepolymerization processes may not employ monomer as the diluent, such asin the case of selected examples of polypropylene production where thepropylene monomer itself acts as the diluent. Nevertheless, the diluentmay mix with fluff and other components in the reactor to form a polymerslurry. In general, the diluent may facilitate circulation of thepolymer slurry in the reactor, heat removal from the polymer slurry inthe reactor, and so on.

The slurry discharge of the reactor typically includes the polymer fluffas well as non-polymer components such as unreacted olefin monomer (andcomonomer), diluent, and so forth. This discharge stream is generallyprocessed, such as by a diluent/monomer recovery system (e.g., flashvessel or separator vessel, purge column, etc.) to separate thenon-polymer components from the polymer fluff. The recovered diluent,unreacted monomer, and other non-polymer components from the recoverysystem may be treated and recycled to the reactor, for example. As forthe recovered polymer (solids), the polymer may be treated to deactivateresidual catalyst, remove entrained or dissolved hydrocarbons, dry thepolymer, and pelletize the polymer in an extruder, and so forth, beforethe polymer is sent to customers.

In some circumstances, to increase capacity of a polyolefinpolymerization line or to achieve certain desired polymercharacteristics, more than one polymerization reactor may be employed,with each reactor having its own set of conditions. In certain examples,the reactors (e.g., loop reactors) may be connected in series, such thatthe polymer slurry from one reactor may be transferred to a subsequentreactor, and so forth, until a polyolefin polymer is produceddischarging from the final or terminal reactor with the desired set ofcharacteristics. The respective reactor conditions including thepolymerization recipe can be set and maintained such that the polyolefin(e.g., polyethylene, polypropylene) polymer product is monomodal,bimodal, or multimodal, and having polyolefin portions of differentdensities, and so on.

The polymerization in a single or multiple reactors is generallyexothermic, or heat-generating, and is typically performed in closedsystems where temperature and pressure can be regulated to controlproduction. As with any such closed system where heat is generated, somemeans should be supplied to remove heat and thus to control thepolymerization temperature. For loop reactors and other polymerizationreactors, a cooling or coolant system is typically used to remove heat.

Variations in reactor feedstocks, utility supplies, and reactionkinetics induce variations in the reactor (polymerization) temperaturewhich may be mitigated by the reactor temperature control scheme and thereactor coolant system. The control scheme and coolant system shouldalso accommodate reactor upsets caused, for example, by undesirable slugfeed of reactants or by rapidly changing heat transfer behavior in afouling reactor.

Unfortunately, problems may be experienced that cause the coolant systemto remove too little heat or too much heat from the reactor. Poortemperature control in the reactor increases the cost to manufacturepolyolefin. In particular, poor temperature control in the reactorresults in a wider design basis for coolant system equipment and thusincreases equipment costs. Furthermore, swings in reactor temperatureimpact reactor stability and can lead to a reactor foul and/or unplannedshutdown. Additionally, polymerization temperature affects theproperties of the polyolefin and thus poor control of reactortemperature cause off-spec production of polyolefin. Moreover,employment of multiple polymerization reactors in a polyolefin reactorsystem may add complexity and cost of the coolant system and reactortemperature control.

The competitive business of polyolefin production drives manufacturersin the continuous improvement of their processes in order to improveoperability and product quality, lower production costs, and so on. Inan industry where billions of pounds of polyolefins are produced peryear, small incremental improvements, such as in reducing capital andoperating costs associated with reactor cooling while maintainingeffective temperature control and product quality, can result in a moreattractive technology and economic benefit including greater pricemargins and netback, and so forth.

SUMMARY OF THE INVENTION

An aspect of the invention relates to a polyolefin reactor temperaturecontrol system including: (a) a first reactor temperature control pathfor: a first control feed stream to split into at least (1) a firstcooler zone feed stream to pass through a first cooler zone to produce afirst cooler zone output stream and (2) a first cooler zone bypassstream; a first treated stream having a first treated stream temperatureand comprising the first cooler zone output stream and the first coolerzone bypass stream; and a first recycle stream comprising the firsttreated stream after the first treated stream has exchanged energy witha first polyolefin reactor. The reactor temperature control systemfurther includes (b) a second reactor temperature control path for: asecond control feed stream to split into at least (1) a second coolerfeed stream to pass through a second cooler zone to produce a secondcooler zone output stream and (2) a second cooler zone bypass stream; asecond treated stream having a second treated stream temperature andcomprising the second cooler zone output stream and the second coolerzone bypass stream; and a second recycle stream comprising the secondtreated stream after the second treated stream has exchanged energy witha second polyolefin reactor. Lastly, included is (c) a sharedtemperature control path configured to: combine the first and secondrecycle streams to form a combined recycle stream; process the combinedrecycle stream through shared system equipment to form a shared outputstream; and split the shared output stream into the first control feedstream and the second control feed stream.

Another aspect of the invention includes a method of controlling reactortemperature, including: splitting a first control feed stream into atleast (1) a first cooler zone feed stream through a first cooler zone toproduce a first cooler zone output stream and (2) a first cooler zonebypass stream; combining the first cooler zone output stream and thefirst cooler zone bypass stream to give a first treated stream having afirst treated stream temperature; recycling a first return streamcomprising the first treated stream after the first treated stream hasexchanged energy with a first polyolefin reactor; splitting a secondcontrol feed stream into at least (1) a second cooler zone feed streamthrough a second cooler zone to produce a second cooler zone outputstream and (2) a second cooler zone bypass stream; combining the secondcooler zone output stream and the second cooler zone bypass stream togive a second treated stream having a second treated stream temperature;recycling a second return stream comprising the second treated streamafter the second treated stream has exchanged energy with a secondpolyolefin reactor; combining the first and second return streams toform a combined return stream; processing the combined return streamthrough shared system equipment to form a shared output stream; andsplitting the shared output stream into the first control feed and thesecond control feed.

Yet another aspect of the invention relates to polyolefin reactorsystem, having a total reactor system production rate, including: afirst polymerization reactor having a first reactor production rate; asecond polymerization reactor having a second reactor production rate;and a reactor temperature control system including a first reactortemperature control path, a second reactor temperature control path, anda shared temperature control path including a pump having a single pumpdischarge rate split between the first reactor temperature control pathand the second temperature control path, wherein a ratio of the totalreactor system production rate to the single pump discharge rate isgreater than 0.004 pounds polyethylene per pound of coolant (treatedwater).

Yet another aspect of the invention relates to a reactor systemincluding: a first polyolefin reactor; a second polyolefin reactor; anda reactor temperature control system having a coolant pump. The coolantpump is configured to: provide coolant supply to the first polyolefinreactor through a first cooler and a first bypass line disposedoperationally in parallel to the first cooler; provide coolant supply tothe second polyolefin reactor through a second cooler and a secondbypass line disposed operationally in parallel to the second cooler; andreceive coolant return from the first polyolefin reactor and coolantreturn from the second polyolefin reactor.

Lastly, yet another aspect of the invention relates to a methodincluding: polymerizing olefin in a first reactor to form a firstpolyolefin; polymerizing olefin in a second reactor to form a secondpolyolefin; providing coolant supply via a coolant pump to the firstreactor through a first cooler and a first bypass line disposedoperationally in parallel to the first cooler; providing coolant supplyvia the coolant pump to the second reactor through a second cooler and asecond bypass line disposed operationally in parallel to the firstcooler; and receiving coolant return at the coolant pump from the firstand second reactors.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent to one of skill in theart upon reading the following detailed description and upon referenceto the drawings in which:

FIG. 1 is a block flow diagram depicting an exemplary polyolefinproduction system for producing polyolefin in accordance withembodiments of the present techniques;

FIG. 2 is a process flow diagram of an exemplary reactor system of thepolyolefin production system of FIG. 1 and not having a CTO on the firstreactor discharge in accordance with embodiments of the presenttechniques;

FIG. 2A is a process flow diagram of an exemplary reactor system of thepolyolefin production system of FIG. 1 and having a CTO on the firstreactor discharge in accordance with embodiments of the presenttechniques;

FIG. 3 is a diagrammatical representation of the exemplary reactorsystems (of FIGS. 1-2A) having a cooling (coolant) system in accordancewith embodiments of the present techniques;

FIG. 4 is a schematic of a reactor of the reactor systems (of FIGS.1-2A) having reactor jackets and employing an exemplary cooling(coolant) system in accordance with embodiments of the presenttechniques;

FIG. 5 is a diagrammatical representation of the exemplary coolingsystem (of FIG. 4) supplying multiple reactors in accordance withembodiments of the present techniques;

FIG. 6 is a process diagram of a portion of the exemplary cooling systemof FIGS. 4-5 in accordance with embodiments of the present techniques;

FIG. 7 is another process diagram of the portion of the exemplarycooling system shown in FIG. 6 in accordance with embodiments of thepresent techniques;

FIG. 8 is a block flow diagram of a method for an exemplary reactortemperature control scheme in accordance with embodiments of the presenttechniques;

FIG. 9 is an exemplary plot of valve openings of three control valvesversus controller output for the three valves in the cooling system inaccordance with embodiments of the present techniques;

FIG. 10 are exemplary plots showing a change in reactor temperature setpoint and the resulting output of the reactor temperature controller andcoolant controller in the cooling system in accordance with embodimentsof the present techniques;

FIG. 11 is a block flow diagram of a method of controlling reactortemperature in a polyolefin reactor system having multiplepolymerization reactors.

FIG. 12 is a diagrammatical representation of the exemplary coolingsystem of FIGS. 4-5 in accordance with embodiments of the presenttechniques; and

FIGS. 13-16 are alternate embodiments of a portion of the exemplarycooling system of FIGS. 4-5 in accordance with embodiments of thepresent techniques.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. To provide a concise description of these embodiments,not all features of an actual implementation are described in thespecification. It should be appreciated that in the development of anysuch actual implementation, as in any engineering or design project,numerous implementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would nevertheless be aroutine undertaking of design, fabrication, and manufacture for those ofordinary skill in the art and having the benefit of this disclosure.

The present techniques accommodate the production of the same ordifferent polyolefins in respective polymerization reactors in series orparallel. The polyolefin produced in the reactors may be the same ordifferent with respect to polymer density, molecular weight, and so on.To improve such production, the present techniques address temperaturecontrol of the multiple polymerization reactors in the reactor system.The techniques address the precision of the temperature control, and theassociated capital and operating costs of the coolant system used toimplement the reactor temperature control.

Example temperature control and associated coolant systems are discussedbelow in Section III. The techniques uniquely employ a common or sharedcoolant pump to provide respective coolant supply to the multiplepolymerization reactors in the polyolefin reactor system.Advantageously, such a unified or shared motive force for coolant supplymay reduce capital and operating costs of the coolant system.

Further, the coolant supply from the shared pump is split into dedicatedcooling for the respective reactors, thereby providing more precisetemperature control of the reactors, such as within a desired toleranceof +/−0.25° F., for example. Thus, the coolant system includes a commonor shared portion (motive force) supplying the reactors, as well asindividual or dedicated portions (cooling and temperature control) forthe respective reactors.

In operation, a control scheme may direct the coolant system to maintainthe reactor temperature at a desired set point. The temperature controlmay involve a cascade control scheme, or in other words, a primarycontroller (i.e., that maintains reactor temperature) that directs aslave controller (i.e., that adjusts coolant temperature). The slavecontroller may send an output to adjust the position of one or morevalves in the coolant system.

To achieve desired polymer characteristics in polyolefin production,more than one polymerization reactor may be employed, with each reactorhaving its own set of conditions. The reactors (e.g., loop reactors) maybe connected in series, such that all or at least a portion of thepolymer fluff slurry from one reactor may be transferred to a subsequentreactor, and so forth, until a polyolefin polymer is produceddischarging from the final or terminal reactor with the desired set ofcharacteristics. The respective reactor conditions including thepolymerization recipe can be set and maintained such that the polyolefin(e.g., polyethylene, polypropylene) polymerized in each respectivereactor may have a different molecular weight, different density, and soon. In the case of two reactors in series, two polyolefin polymers(e.g., one polymerized in the first reactor and the other polymerized inthe second reactor), each having a different molecular weight fractionor different density, for instance, may be combined into one polymerproduct discharging from the second (final) reactor.

Thus, in polyolefin production with polymerization reactors in series,the reactors can be operated to produce different polyolefin polymers ineach reactor. For example, the olefin monomer may be polymerized in thefirst reactor to produce a high molecular-weight polyolefin and having alow or high polymer density, and the olefin monomer polymerized in thesecond reactor to produce a low molecular-weight polyolefin and having alow or high polymer density. On the other hand, the olefin monomer maybe polymerized in the first reactor to produce a low molecular-weightpolyolefin and having a low or high polymer density, and the olefinmonomer polymerized in the second reactor to produce a highmolecular-weight polyolefin and having a low or high polymer density.Further, similar molecular weight polyolefin may be produced in eachreactor but with the polyolefin density or other properties beingdifferent in each reactor.

In a certain examples with two polymerization reactors (e.g., loopreactors) in series, a low molecular-weight high-density polyethylene(LMW HDPE) is produced in one reactor and a high molecular-weight linearlow-density polyethylene (HMW LLDPE) produced in the other reactor.Thus, the combined final product is a bimodal polyethylene dischargingfrom the final (second reactor). A chain transfer agent (e.g., hydrogen)is fed to the reactor polymerizing the LMW HDPE to terminate polymerchain growth in the addition polymerization to facilitate production ofthe LMW HDPE in that reactor. Therefore, as may be deduced from theforegoing discussion, the cooling requirements for the reactors may varyconsiderably.

Indeed, as for maintaining the polymerization temperature in thereactors, the cooling requirements of the respective reactors may varyconsiderably depending on the type or grade and amount of polyolefinbeing produced. In other words, the amount of heat generated in areactor and, thus, the cooling required may be different acrossdifferent grades or types and production rates of polyolefin, andbetween two reactors in a given reactor system, such as in bimodalproduction. In fact, the amount of chain transfer agent added, forexample, and the degree of polymerization, may generate more or lessheat of reaction. Again, the present reactor temperature control andassociated coolant system are discussed in more detail below in SectionIII.

Lastly, while the present discussion may focus on two reactors inseries, the present techniques may be applicable to more than tworeactors in series. Further, the techniques may apply to two or morereactors in parallel, or any combinations of series and parallelreactors. Further, various combinations of molecular weights andcomonomer additions in monomodal, bimodal, or multimodal polyolefin(e.g., polyethylene, polypropylene, etc.) may be applicable.

I. Polyolefin Production Overview

Turning now to the drawings, and referring initially to FIG. 1, a blockdiagram depicts an exemplary production system 10 for producingpolyolefin such as polyethylene, polypropylene, and their copolymers,etc. The exemplary production system 10 is typically a continuousoperation but may include both continuous and batch systems. Anexemplary nominal capacity for the exemplary production system 10 isabout 600-1600 million pounds of polyolefin produced per year. Exemplaryhourly design rates are approximately 65,000 to 200,000 pounds ofpolymerized/extruded polyolefin per hour. It should be emphasized,however, that the present techniques apply to polyolefin manufacturingprocesses including polyethylene production systems having nominalcapacities and design rates outside of these exemplary ranges.

Various suppliers 12 may provide reactor feedstocks 14 to the productionsystem 10 via pipelines, ships, trucks, cylinders, drums, and so forth.The suppliers 12 may include off-site and/or on-site facilities,including olefin plants, refineries, catalyst plants, and the like.Examples of possible feedstocks include olefin monomers and comonomers(such as ethylene, propylene, butene, hexene, octene, and decene),diluents (such as propane, isobutane, n-butane, n-hexane, andn-heptane), chain transfer agents (such as hydrogen), catalysts (such asZiegler-Natta catalysts, chromium catalysts, and metallocene catalysts)which may be heterogeneous, homogenous, supported, unsupported, andco-catalysts such as, triethylboron, organoaluminum compounds, methylaluminoxane (MAO), triethylaluminum (TEAl), borates, TiBAL, etc., andactivators such as solid super acids, and other additives. In the caseof ethylene monomer, exemplary ethylene feedstock may be supplied viapipeline at approximately 800-1450 pounds per square inch gauge (psig)at 45-65° F. Exemplary hydrogen feedstock may also be supplied viapipeline, but at approximately 900-1000 psig at 90-110° F. Of course, avariety of supply conditions may exist for ethylene, hydrogen, and otherfeedstocks 14.

The suppliers 12 typically provide feedstocks 14 to a reactor feedsystem 16, where the feedstocks 14 may be stored, such as in monomerstorage and feed tanks, diluent vessels, catalyst tanks, co-catalystcylinders and tanks, and so forth. In the case of ethylene monomer feed,the ethylene may be fed to the polymerization reactors withoutintermediate storage in the feed system 16 in certain embodiments. Inthe feed system 16, the feedstocks 14 may be treated or processed priorto their introduction as feed 18 into the polymerization reactor system20. For example, feedstocks 14, such as monomer, comonomer, and diluent,may be sent through treatment beds (e.g., molecular sieve beds, aluminumpacking, etc.) to remove catalyst poisons. Such catalyst poisons mayinclude, for example, water, oxygen, carbon monoxide, carbon dioxide,and organic compounds containing sulfur, oxygen, or halogens. The olefinmonomer and comonomers may be liquid, gaseous, or a supercritical fluid,depending on the type of reactor being fed. Also, it should be notedthat typically only a relatively small amount of fresh make-up diluentas feedstock 14 is utilized, with a majority of the diluent fed to thepolymerization reactor recovered from the reactor effluent.

The feed system 16 may prepare or condition other feedstocks 14, such ascatalysts, for addition to the polymerization reactors. For example, acatalyst may be prepared and then mixed with diluent (e.g., isobutane orhexane) or mineral oil in catalyst preparation tanks. Further, the feedsystem 16 typically provides for metering and controlling the additionrate of the feedstocks 14 into the polymerization reactor to maintainthe desired reactor stability and/or to achieve the desired polyolefinproperties or production rate. Furthermore, in operation, the feedsystem 16 may also store, treat, and meter recovered reactor effluentfor recycle to the reactor. Indeed, operations in the feed system 16generally receive both feedstock 14 and recovered reactor effluentstreams.

In total, the feedstocks 14 and recovered reactor effluent are processedin the feed system 16 and fed as feed streams 18 (e.g., streams ofmonomer, comonomer, diluent, catalysts, co-catalysts, hydrogen,additives, or combinations thereof) to the reactor system 20. Asdiscussed below, the streams 18 may be delivered in feed conduits to thereactor which tap into the wall of the polymerization reactor in thereactor system 20. Moreover, a given feed system 16 may be dedicated toa particular reactor or to multiple reactors disposed/operated in seriesor parallel. Further, a feed system 16 may receive recycle components(e.g., diluent) from one or more downstream processing systems.

The reactor system 20 may have one or more reactor vessels, such asliquid-phase or gas-phase reactors. If multiple reactors are employed,the reactors may be arranged in series, in parallel, or in othercombinations or configurations. Moreover, multiple reactors arranged andoperated in series may be shifted in operation to a parallel orindependent operation.

In the polymerization reactor vessels, one or more olefin monomers andoptionally comonomers are polymerized to form a product polymerparticulates, typically called fluff or granules. In one example, themonomer is ethylene and the comonomer is 1-hexene. In another example,the monomer is propylene and the comonomer is ethylene. The fluff maypossess one or more melt, physical, rheological, and/or mechanicalproperties of interest, such as density, melt index (MI), molecularweight, copolymer or comonomer content, modulus, and the like. Thereaction conditions, such as temperature, pressure, flow rate,mechanical agitation, product takeoff, component concentrations,catalyst type, polymer production rate, and so forth, may be selected toachieve the desired fluff properties.

In addition to the one or more olefin monomers and comonomers, acatalyst that facilitates polymerization of the ethylene monomer istypically added to the reactor. The catalyst may be a particle suspendedin the fluid medium within the reactor. In general, Ziegler catalysts,Ziegler-Natta catalysts, metallocene catalysts, chromium catalysts,nickel catalysts, post-metallocene and other well-known polyolefincatalysts, as well as co-catalysts, may be used. Typically, anolefin-free diluent or mineral oil, for example, is used in thepreparation and/or delivery of the catalyst in a feed conduit that tapsinto the wall of the polymerization reactor. Further, diluent may be fedinto the reactor, typically a liquid-phase reactor.

The diluent may be an inert hydrocarbon that is liquid at reactionconditions, such as isobutane, propane, n-butane, n-pentane, i-pentane,neopentane, n-hexane, n-heptane, cyclohexane, cyclopentane,methylcyclopentane, ethylcyclohexane, and the like. The purpose of thediluent is generally to suspend the catalyst particles and polymerwithin the reactor. Diluent, as indicated, may also be used for reactoror line flushes to mitigate plugging or fouling, to facilitate flow ofthe polymer slurry in lines, and so on. Moreover, in examples ofpolypropylene production, the propylene monomer itself may act as adiluent.

A motive device may be present within each of the one or more reactorsin the reactor system 20. For example, within a liquid-phase reactor,such as a loop slurry reactor, an impeller may create a mixing zonewithin the fluid medium. The impeller may be driven by a motor to propelthe fluid medium as well as any catalyst, polyolefin fluff, or othersolid particulates suspended within the fluid medium, through the closedloop of the reactor. Similarly, within a gas-phase reactor, such as afluidized bed reactor or plug flow reactor, one or more paddles orstirrers may be used to mix the solid particles within the reactor.Lastly, as discussed in more detail in Section III below, the reactorsystem 20 typically includes a coolant system to facilitate control oftemperature in the polymerization reactors.

The discharge of polyolefin fluff product slurry 22 of the reactors fromsystem 20 may include the polymer polyolefin fluff as well asnon-polymer components, such as diluent, unreacted monomer/comonomer,and residual catalyst. In construction of the reactors in certainembodiments, a discharge nozzle and conduit may be installed (e.g.,welded) at a tap or hole cut into the reactor wall. The discharge of thefluff product slurry 22 exiting the reactor system (e.g., the finalreactor in a series of reactors) through the discharge nozzle may besubsequently processed, such as by a diluent/monomer recovery system 24.The fluff product slurry 22 may also be called a reactor productdischarge slurry, a reactor product discharge, or a reactor discharge,etc. Thus, depending on context, a “reactor discharge” may refer to thefluff slurry exiting the reactor and/or to the physical configuration(e.g., reactor wall tap or opening, nozzle, conduit, valve if any, etc.)discharging the fluff slurry.

Furthermore, it should be noted that the liquid (e.g., diluent) in fluffproduct slurry 22 generally partially or fully vaporizes through a flashline including a flash line heater (not shown) downstream of the reactorin route to the diluent/monomer recovery system 24. As discussed below,such vaporization may be due to decreasing pressure through the flashline, and/or due to heat added by the flash line heater (e.g., a conduitwith a steam or steam condensate jacket). The diluent/monomer recoverysystem 24 may process the fluff product slurry 22 (whether the liquid inthe slurry 22 is partially or fully vaporized in the flash line) fromthe reactor system 20 to separate non-polymer components 26 (e.g.,diluent and unreacted monomer) from the polymer fluff 28.

A fractionation system 30 may process the untreated recoverednon-polymer components 26 (e.g., diluent/monomer) to remove undesirableheavy and light components and to produce olefin-free diluent, forexample. Fractionated product streams 32 may then return to the reactorsystem 20 either directly (not shown) or via the feed system 16. Sucholefin-free diluent may be employed in catalyst preparation/delivery inthe feed system 16 and as reactor or line flushes in the reactor system20.

A portion or all of the non-polymer components 26 may bypass thefractionation system 30 and more directly recycle to the reactor system(not shown) or the feed system 16, as indicated by reference numeral 34.In certain embodiments, up to 80-95% of the diluent discharged from thereactor system 20 bypasses the fractionation system 30 in route to thepolymerization feed system 16 (and ultimately the reactor system 20). Ofcourse, in other embodiments, no diluent bypasses the fractionationsystem 30, or in other words, there is no direct recycle of diluent tothe reactors. Moreover, although not illustrated, polymer granulesintermediate in the recovery system 24 and typically containing activeresidual catalyst may be returned to the reactor system 20 for furtherpolymerization, such as in a different type of reactor or underdifferent reaction conditions.

The polyolefin fluff 28 discharging from the diluent/monomer recoverysystem 24 may be extruded into polyolefin pellets 38 in an extrusionsystem 36. In the extrusion system 36, the fluff 28 is typicallyextruded to produce polymer pellets 38 with the desired mechanical,physical, and melt characteristics. An extruder/pelletizer receives theextruder feed including one or more fluff products 28 and whateveradditives have been added. Extruder feed may include additives added tothe fluff products 28 to impart desired characteristics to the extrudedpolymer pellets 38. The extruder/pelletizer heats and melts the extruderfeed which then may be extruded (e.g., via a twin screw extruder)through a pelletizer die under pressure to form polyolefin pellets 38.Such pellets are typically cooled in a water system disposed at or nearthe discharge of the pelletizer.

A loadout system 39 may prepare the polyolefin pellets 38 for shipmentin to customers 40. In general, the polyolefin pellets 38 may betransported from the extrusion system 36 to a product loadout area 39where the pellets 38 may be stored, blended with other pellets, and/orloaded into railcars, trucks, bags, and so forth, for distribution tocustomers 40. Polyolefin pellets 38 shipped to customers 40 may includelow density polyethylene (LDPE), linear low density polyethylene(LLDPE), medium density polyethylene (MDPE), high density polyethylene(HDPE), enhanced polyethylene, isotactic polypropylene (iPP),syndiotactic polypropylene (sPP), including various copolymers, and soon. The polymerization and diluent recovery portions of the polyolefinproduction system 10 may be called the “wet” end 42 or alternatively“reaction” side of the process 10. The extrusion 36 and loadout 39systems of the polyolefin production system 10 may be called the “dry”end 44 or alternatively “finishing” side of the polyolefin process 10.Moreover, while the polyolefin pellets 38 discharging from the extrusionsystem 36 may be stored and blended in the loadout area 39, thepolyolefin pellets 38 are generally not altered by the loadout system 39prior to being sent to the customer 40.

Polyolefin pellets 38 may be used in the manufacturing of a variety ofproducts, components, household items and other items, includingadhesives (e.g., hot-melt adhesive applications), electrical wire andcable, agricultural films, shrink film, stretch film, food packagingfilms, flexible food packaging, milk containers, frozen-food packaging,trash and can liners, grocery bags, heavy-duty sacks, plastic bottles,safety equipment, carpeting, coatings, toys and an array of containersand plastic products. To form the end-products or components, thepellets 38 are generally subjected to processing, such as blow molding,injection molding, rotational molding, blown film, cast film, extrusion(e.g., sheet extrusion, pipe and corrugated extrusion,coating/lamination extrusion, etc.), and so on. Ultimately, the productsand components formed from polyolefin pellets 38 may be furtherprocessed and assembled for distribution and sale to the consumer. Forexample, extruded pipe or film may be packaged for distribution to thecustomer, or a fuel tank comprising polyethylene may be assembled intoan automobile for distribution and sale to the consumer, and so on.

Process variables in the polyolefin production system 10 may becontrolled automatically and/or manually via valve configurations,control systems, and so on. In general, a control system, such as aprocessor-based system, may facilitate management of a range ofoperations in the polyolefin production system 10, such as thoserepresented in FIG. 1. Polyolefin manufacturing facilities may include acentral control room or location, as well as a central control system,such as a distributed control system (DCS) and/or programmable logiccontroller (PLC). The reactor system 20 typically employs aprocessor-based system, such as a DCS, and may also employ advancedprocess control known in the art. The feed system 16, diluent/monomerrecovery 24, and fractionation system 30 may also be controlled by theDCS. In the dry end of the plant, the extruder and/or pellet loadingoperations may also be controlled via a processor-based system (e.g.,DCS or PLC). Moreover, in the controls systems, computer-readable mediamay store control executable code to be executed by associatedprocessors including central processing units, and the like. Such codeexecutable by the processor may include logic to facilitate theoperations described herein.

The DCS and associated control system(s) in the polyolefin productionsystem 10 may include the appropriate hardware, software logic and code,to interface with the various process equipment, control valves,conduits, instrumentation, etc., to facilitate measurement and controlof process variables, to implement control schemes, to performcalculations, and so on. A variety of instrumentation known to those ofordinary skill in the art may be provided to measure process variables,such as pressure, temperature, flow rate, and so on, and to transmit asignal to the control system, where the measured data may be read by anoperator and/or used as an input in various control functions. Dependingon the application and other factors, indication of the processvariables may be read locally or remotely by an operator, and used for avariety of control purposes via the control system.

A polyolefin manufacturing facility typically has a control room fromwhich the plant manager, engineer, technician, supervisor and/oroperator, and so on, monitors and controls the process. When using aDCS, the control room may be the center of activity, facilitating theeffective monitoring and control of the process or facility. The controlroom and DCS may contain a Human Machine Interface (HMI), which is acomputer, for example, that runs specialized software to provide auser-interface for the control system. The HMI may vary by vendor andpresent the user with a graphical version of the remote process. Theremay be multiple HMI consoles or workstations, with varying degrees ofaccess to data.

II. Polymerization Reactor System

As discussed above, the reactor system 20 may include one or morepolymerization reactors, which may in turn be of the same or differenttypes. Furthermore, with multiple reactors, the reactors may be arrangedserially or in parallel. Whatever the reactor types in the reactorsystem 20, a polyolefin particulate product, generically referred to as“fluff” herein, is produced. To facilitate explanation, the followingexamples are limited in scope to specific reactor types believed to befamiliar to those skilled in the art and to combinations. To one ofordinary skill in the art using this disclosure, however, the presenttechniques are applicable to more complex reactor arrangements, such asthose involving additional reactors, different reactor types, and/oralternative ordering of the reactors or reactor types, as well asvarious diluent and monomer recovery systems and equipment disposedbetween or among the reactors, and so on. Such arrangements areconsidered to be well within the scope of the present invention.

One reactor type includes reactors within which polymerization occurswithin a liquid phase. Examples of such liquid phase reactors includeautoclaves, boiling liquid-pool reactors, loop slurry reactors (verticalor horizontal), and so forth. For simplicity, a loop slurry reactorwhich produces polyolefin, such as polyethylene or polypropylene, isdiscussed in the present context though it is to be understood that thepresent techniques may be similarly applicable to other types of liquidphase reactors.

FIG. 2 depicts an exemplary polymerization reactor system 20 (of FIG. 1)as having two loop slurry (polymerization) reactors 50A, 50B disposedand operated in series. Additional loop reactors or other reactors(e.g., gas phase reactors) may be disposed in series or parallel in theillustrated combination. Moreover, in embodiments, the reactors 50A, 50Bmay be shifted to a parallel operation, and/or processing equipment maybe disposed between the two loop reactors 50A, 50B, and so on. Thepresent techniques contemplate aspects of a variety of reactor systemconfigurations such as those also disclosed in U.S. Patent ApplicationPublication No. 2011/0288247 which is incorporated by reference hereinin its entirety. The processing equipment disposed between the reactors50A, 50B, if so disposed, may remove diluent, solids, light components,hydrogen, and so forth, from the transfer slurry 21 for recycle to thefirst reactor 50A and/or to a recovery system, and the like.

A loop slurry reactor 50A, 50B is generally composed of segments of pipeconnected by smooth bends or elbows. The representation of the loopreactors 50A, 50B in FIG. 2 is simplified, as appreciated by the skilledartisan. Indeed, an exemplary reactor 50A, 50B configuration may includeeight to sixteen or other number of jacketed vertical pipe legs (see,e.g., FIG. 4) approximately 24 inches in diameter and approximately 200feet in length, connected by pipe elbows at the top and bottom of thelegs. FIG. 2 shows a four leg segment reactor arranged vertically. Itcould also be arranged horizontally. The reactor jackets 52 are normallyprovided to remove heat from the exothermic polymerization viacirculation of a cooling medium or coolants, such as treated water,through the reactor jackets 52. See Section III below for a discussionof the cooling system and reactor temperature control.

The reactors 50A, 50B may be used to carry out polyolefin (e.g.,polyethylene, polypropylene) polymerization under slurry conditions inwhich insoluble particles of polyolefin are formed in a fluid medium andare suspended as slurry until removed. A respective motive device, suchas pump 54A, 54B, circulates the fluid slurry in each reactor 50A, 50B.An example of a pump 54A, 54B is an in-line axial flow pump with thepump impeller disposed within the interior of the reactor 50A, 50B tocreate a turbulent mixing zone within the fluid medium. The impeller mayalso assist in propelling the fluid medium through the closed loop ofthe reactor at sufficient speed to keep solid particulates, such as thecatalyst or polyolefin product, suspended within the fluid medium. Theimpeller may be driven by a motor 56A, 56B or other motive force.

The fluid medium within each reactor 50A, 50B may include olefinmonomers and comonomers, diluent, co-catalysts (e.g., alkyls,triethylboron, TiBAL, TEAl, methyl aluminoxane or MAO, borates, TEB,etc.), activator supports like solid super acids, molecular weightcontrol agents (e.g., hydrogen), and any other desired co-reactants oradditives. Such olefin monomers and comonomers are generally 1-olefinshaving up to 10 carbon atoms per molecule and typically no branchingnearer the double bond than the 4-position. Examples of monomers andcomonomers include ethylene, propylene, butene, 1-pentene, 1-hexene,1-octene, and 1-decene. Again, typical diluents are hydrocarbons whichare inert and liquid under reaction conditions, and include, forexample, isobutane, propane, n-butane, n-pentane, i-pentane, neopentane,n-hexane, n-heptane, cyclohexane, cyclopentane, methylcyclopentane,ethylcyclohexane, and the like. These components are added to thereactor interior via inlets or conduits at specified locations, such asdepicted at feed streams 53A, 53B, which generally corresponds to one ofthe feed streams 18 of FIG. 1.

Likewise, a catalyst, such as those previously discussed, may be addedto the reactor 50A via a conduit at a suitable location, such asdepicted at feed stream 55, which may include a diluent carrier andwhich also generally corresponds to one of the feed streams 18 ofFIG. 1. Again, the conduits that feed the various components tie-in(i.e., flange or weld) to the reactor 50. In the illustrated embodiment,catalyst feed 55 is added to the first reactor 50A in series but not tothe second reactor 50B. However, active catalyst may discharge in afluff slurry 21 from the first reactor 50A to the second reactor 50B.Moreover, while not depicted, a fresh catalyst may be added to thesecond reactor 50B in certain embodiments. In total, the addedcomponents including the catalyst and other feed components generallycompose a fluid medium within the reactor 50A, 50B in which the catalystis a suspended particle.

The reaction conditions, such as temperature, pressure, and reactantconcentrations, in each reactor 50A, 50B are regulated to facilitate thedesired properties and production rate of the polyolefin in the reactor,to control stability of the reactor, and the like. Temperature istypically maintained below that level at which the polymer product wouldsignificantly go into solution, swell, soften, or become sticky. Asindicated, due to the exothermic nature of the polymerization reaction,a cooling fluid or coolant may be circulated through jackets 52 aroundportions of the loop slurry reactor 50A, 50B (see Section III below) toremove excess heat, thereby maintaining the temperature within thedesired range, generally between 150° F. to 250° F. (65° C. to 121° C.).Likewise, pressure in each loop reactor 50A, 50B may be regulated withina desired pressure range, generally 100 to 800 psig, with a range of 450to 700 psig being typical. Of course, the reactor cooling andtemperature control techniques disclosed herein may be applicable tolower-pressure polyolefin processes, such as those with reactorsoperating in a typical range of 50 psig to 100 psig, and with hexane asa common diluent, for example.

As the polymerization reaction proceeds within each reactor 50A, 50B,the monomer (e.g., ethylene) and comonomers (e.g., 1-hexene) polymerizeto form polyolefin (e.g., polyethylene) polymers that are substantiallyinsoluble in the fluid medium at the reaction temperature, therebyforming a slurry of solid particulates within the medium. These solidpolyolefin particulates may be removed from each reactor 50A, 50B via areactor discharge. In the illustrated embodiment of FIG. 2, a transferslurry 21 is discharged from the first reactor 50A, and a product slurry22 is discharged from the second reactor 50B.

For the transfer slurry 21 and product slurry 22, the reactor dischargemay be (1) an intermittent discharge such as a settling leg, pulsatingon/off valve, and so on, or (2) a continuous discharge such ascontinuous take-off (CTO) which has a modulating valve, and so forth. Asfor a continuous discharge of the transfer slurry 21 from the firstreactor 50A, a continuous discharge on the first reactor may be a CTO,or may be a continuous discharge without a modulating valve (but with anisolation valve, for example), and so on. The pressure differentialbetween the discharge of the first loop reactor pump 54A and the suctionof the second loop reactor pump 54B may provide a motive force for thetransfer of transfer slurry 21 from the first loop reactor 50A to thesecond loop reactor 50B. Moreover, the pump suction, whether of thefirst pump 54A or second pump 54B, may be considered upstream of thepump in pipe length of the respective loop in the range of about 0.5meter to 50 meters (e.g., 0.5, 1, 5, 15, 25, 50 meters, or values inbetween).

Again, in certain examples, the two loop reactors 50A, 50B may beoperated in series and such that the polyolefin fluff in the fluffslurry 22 discharging from the second reactor 50B is monomodal orbimodal. In certain cases of monomodal production, the reactor operatingconditions may be set such that essentially the same polyolefin orsimilar polyolefin is polymerized in each reactor 50A, 50B. On the otherhand, in monomodal production in terms of molecular weight, theconditions in the reactor may be the same or similar such as with regardto hydrogen concentration but different in terms of comonomerconcentration, for example, so to produce polyolefin with similarmolecular weight but different polymer density in each reactor.

In the case of bimodal production, the reactor operating conditions maybe set such that the polyolefin polymerized in the first reactor 50A isdifferent than the polyolefin polymerized in the second reactor 50B.Thus, with two reactors, a first polyolefin produced in the first loopreactor 50A and the second polyolefin produced in the second loopreactor 50B may combine to give a bimodal polyolefin or a monomodalpolyolefin. Further, again, whether monomodal or bimodal, i.e., in termsof molecular weight, the first polyolefin produced in the first loopreactor 50A and the second polyolefin produced in the second loopreactor 50B may have different polymer densities, for example.

Operation of the two loop reactors 50A, 50B may include feeding morecomonomer to the first polymerization reactor than to the secondpolymerization reactor, or vice versa. The operation may also includefeeding more chain transfer agent (e.g., hydrogen) to the secondpolymerization reactor than the second reactor, or vice versa. Ofcourse, the same amount of comonomer and/or the same amount of chaintransfer agent (e.g., hydrogen) may be fed to each reactor 50A, 50B.Further, the same or different comonomer concentration may be maintainedin each reactor 50. Likewise, the same or different chain transfer agent(e.g., hydrogen) concentration may be maintained in each reactor 50A,50B.

Furthermore, the first polyolefin (i.e., polyolefin polymerized in thefirst reactor 50A) may have a first range for a physical property, andthe second polyolefin (i.e., polyolefin polymerized in the secondreactor 50B) may have a second range for the physical property. Thefirst range and the second range may be the same or different. Exemplaryphysical properties may include polyolefin density, comonomerpercentage, short chain branching amount, molecular weight, viscosity,melt index, melt flow rate, crystallinity, and the like.

As indicated, the polyolefin product fluff slurry 22 discharges from thesecond reactor 50B and is subjected to downstream processing, such as ina diluent/monomer recovery system 24. The product fluff slurry 22 maydischarge through a settling leg, a continuous take-off (CTO), or othervalve configurations. The product fluff slurry 22 may dischargeintermittently such as through a settling leg configuration or pulsatingon/off valve, or instead may discharge continuously such as through aCTO.

In operation, depending on the positioning of the discharge on thereactor, for example, a discharge slurry 22 having a greater solidsconcentration than the slurry circulating in the reactor 50B may berealized with a discharge configuration having an isolation valve (Ramvalve) alone, or having a CTO configuration with an isolation valve (Ramvalve) and modulating valve 25. A Ram valve in a closed position maybeneficially provide a surface that is flush with the inner wall of thereactor to preclude the presence of a cavity, space, or void for polymerto collect when the Ram valve is in the closed position. Exemplary CTOconfigurations and control, and other discharge configurations, may befound in the aforementioned U.S. Patent Application Publication No.2011/0288247, and in U.S. Pat. No. 6,239,235 which is also incorporatedherein by reference in its entirety.

In the illustrated embodiment, the product fluff slurry 22 dischargesthrough a CTO. In certain examples, a CTO has a Ram valve at the reactor50B wall, and a modulating flow control valve 25 (e.g., v-ball controlvalve) on the discharge conduit. Again, however, in an alternateembodiment, the product fluff slurry 22 may discharge through a settlingleg configuration, for example, in lieu of a CTO.

In the embodiment of FIG. 2, a transfer fluff slurry 21 discharges fromthe first loop reactor 50A to the second loop reactor 50B via a transferline 21L. The contents of transfer fluff slurry 21 may be representativeof the contents of the first loop reactor 50A. However, the solidsconcentration may be greater in the transfer slurry 21 than in the firstloop reactor 50A, depending on the positioning of the inlet of thetransfer line 21L on the first loop reactor 50A, for example, and otherconsiderations. The transfer fluff slurry 21 may discharge from thefirst loop reactor 50A into the transfer line 21L through a settlingleg, an isolation valve (e.g., a Ram valve), a continuous take-off(which as indicated the CTO has an isolation or Ram valve and amodulating valve), or other valve configuration.

As indicated, a variety of discharge configurations are contemplated fora continuous discharge. Employment of an isolation valve (e.g.,full-bore Ram valve) without an accompanying modulating valve mayprovide for continuous discharge of slurry from the loop reactor.Further, a CTO is defined as a continuous discharge having at least amodulating flow valve, and provides for a continuous discharge of slurryfrom the loop reactor. In certain examples, a CTO is further defined asa continuous discharge having an isolation valve (e.g., Ram valve) atthe reactor wall and a modulating valve (e.g., v-ball valve) on thedischarge conduit at the reactor.

In the illustrated embodiment, the discharge of the transfer slurry 21from the first loop reactor 50A is continuous and not directlymodulated. A CTO or settling leg is not employed. Instead, the transferslurry 21 discharges through an isolation valve (e.g., Ram valve) (notshown) on the transfer line 21L at the reactor wall and without amodulating valve in this example. In a particular example, the transferslurry 21 discharges through a full-bore Ram valve maintained in afull-open position, and not additionally through a modulating valve. Inalternate embodiments (not illustrated, a modulating valve may bedisposed downstream on the transfer line 21L, or a CTO with itsmodulating valve may be situated at the transfer slurry 21 discharge ofthe first reactor 50A. If so included, the modulating valve may controlflow rate of the transfer slurry 21 and facilitate control of pressurein the first loop reactor 50A. Moreover, a modulating valve or a CTO andits modulating valve may be disposed to facilitate control of the firstreactor 50A discharge when the two reactors 50A and 50B are shifted inoperation to parallel performance, for instance.

Nevertheless, in the various embodiments, an isolation (e.g., Ram) valveis typically disposed on the discharge at the wall of the first loopreactor 50A. The Ram valve may provide for isolation of the transferline 21L from the loop reactor 50A when such isolation is desired. A Ramvalve may also be positioned at the outlet of the transfer line 21L atthe wall of the second loop reactor 50B to provide for isolation of thetransfer line 21L from the second loop reactor 50B when such isolationis desired. It may be desired to isolate the transfer line 21L from thefirst and second loop reactors 50A, 50B during maintenance or downtimeof the reactor system 20, or when an alternate discharge or transferline from the first reactor 50A is placed in service, and so on. Theoperation or control of the Ram valves may be manual,hydraulic-assisted, air-assisted, remote, automated, and so on. Thetransfer line 21L may be manually removed from service (e.g., manuallyclosing the Ram valves) or automatically removed (e.g., via a controlsystem automatically closing the Ram valves) from service.

In the illustrated embodiment, control of pressure (and throughput) inthe first loop reactor 50A and the second loop reactor 50B may befacilitated by operation of the CTO flow control valve 25. In someexamples, the pressure in the first loop reactor 50A may float on thepressure in the second loop reactor 50B. The reactors 50A, 50B may bemaintained at the same, similar, or different pressure. Pressureelements or instruments may be disposed on the reactors 50A, 50B and onthe transfer line 21L. Further, other process variable elements orinstruments indicating temperature, flow rate, slurry density, and soforth, may also be so disposed.

Such instrumentation may include a sensor or sensing element, atransmitter, and so forth. For a pressure element, the sensing elementmay include a diaphragm, for example. For a temperature element orinstrument, the sensing element may include a thermocouple, a resistancetemperature detector (RTD), and the like, of which may be housed in athermowell, for instance. Transmitters may convert a received analogsignal from the sensing element to a digital signal for feed ortransmission to a control system, for example. The various instrumentsmay have local indication of the sense variable. For instance, apressure element or instrument may be or have a local pressure gauge anda temperature element or instrument may be or have a local temperaturegauge, both of which may be read locally by an operator or engineer, forexample.

The inlet position of the transfer line 21L may couple to the first loopreactor 50A on the discharge side of the circulation pump 54A in thefirst loop reactor 50A. The outlet position of the transfer line 21L maycouple to the second loop reactor on the suction side of the circulationpump 54B in the second loop reactor 50B. Such a configuration mayprovide a positive pressure differential (i.e., a driving force) forflow of transfer slurry 21 through the transfer line 21L from the firstloop reactor 50A to the second loop reactor 50B. In one example, atypical pressure differential (provided from the discharge of the firstpump 54A to the suction of the second pump 54B) is about 20 pounds persquare inch (psi). Again, that pump suction, whether of the first pump54A or second pump 54B, may be considered upstream of the pump in linearloop pipe length in the range of about 0.5 meter to 50 meters (e.g.,0.5, 1, 5, 15, 25, 50 meters, or values therebetween). FIG. 2A depictsan exemplary polymerization reactor system 20 in which unlike FIG. 2,the first reactor 50A has a CTO discharge having a modulating valve 27for the transfer slurry 21 discharge.

III. Cooling System for Reactor Temperature Control

Turning now to FIG. 3, an exemplary reactor system 20 of FIGS. 1 and 2is depicted having two polymerization reactors 50A and 50B, and a sharedor common cooling system 58. As shown in FIG. 3 and discussed in moredetail below with respect to subsequent figures, the coolant or coolingsystem 58 provides a coolant supply 60A to reactor 50A, and a coolantsupply 60B to reactor 50B. The cooling system 58 receives a coolantreturn 62A from reactor 50A, and a coolant return 62B from reactor 50B.The coolant removes heat from the reactors and, therefore, the coolantreturn is generally greater in temperature than the coolant supply. Thecooling system 58 processes the coolant return 62A and 62B to providethe cooled coolant supply 60A and 60B to the reactors.

In general, the cooling system 58 removes heat from polymerizationreactors, such as loop reactors and other polymerization reactor typeshaving a reactor jacket or other coolant flow path. The coolant is acooling medium such as treated water. The cooling system 58 discharges acoolant supply to the reactor, i.e., to the reactor heat-transferjackets, and receives a coolant return from the reactor, i.e., from thereactor heat-transfer jackets.

FIG. 4 is the first loop reactor 50A (having eight legs in thisexample). In the illustrated embodiment of FIG. 4, the cooling system 58discharges a coolant supply 60A to the first loop reactor 50A, and acoolant supply 60B to the second loop reactor 50B (not shown). Thecooling system 58 receives and processes a coolant return 62A from thefirst loop reactor 50A, and a coolant return 62B from the second loopreactor 50B. The coolant flowing in the reactor jackets 52 absorbs heatfrom the reactor contents through the reactor wall. The cooling system58 may also be configured to remove heat from additional polymerization(e.g., loop) reactors in addition to loop reactors 50A and 50B.

For the sake of clarity, FIG. 4 depicts the first loop reactor 50A butnot the second loop reactor 50B or additional reactors. However, thesecond loop reactor 50B (and additional reactors) may be similar orsubstantially identical in configuration as reactor 50A with respect tothe cooling system 58. Therefore, while the discussion at times mayfocus on the first loop reactor 50A, the discussion may equally apply tothe second loop reactor 50B and to additional polymerization or loopreactors that employ the cooling system 58.

The illustrated embodiment of FIG. 4 depicts a counter-current flowscheme of coolant through the reactor jackets 52. In embodiments, thecooling system 58 removes heat from the loop reactor 50A via the reactorjackets 52. An example configuration of the reactor jackets 52 for agiven reactor is two counter-current double-pipe exchangers operated inparallel, with the inner pipe (the reactor) having an approximate 22inch internal diameter, and the outer pipe (the jacket) having anapproximate 28 inch internal diameter. In this example, the total heattransfer area of the reactor jackets 52 for one loop reactor is about5,000 square feet for a 4-leg reactor and 10,000 square feet for a 8-legreactor, for instance. Of course, other jacket configurations, sizes,and heat transfer areas can be accommodated with the present techniques.

As discussed above, the loop reactor 50A as shown in FIG. 4 may be usedto carry out polyolefin polymerization under slurry conditions in whichinsoluble particles of polyolefin are formed in a fluid medium and aresuspended as slurry until removed. The loop reactor 50A is generallycomposed of segments of pipe connected by smooth bends or elbows. Amotive device, such as pump 54A, circulates the fluid slurry in thereactor 50A. An example of a pump 54A is an in-line axial flow pump withthe pump impeller disposed within the interior of the reactor 50A. Theimpeller propels the reactor slurry through the closed loop of thereactor, as depicted by arrows, at sufficient speed to keep solidparticulates, such as catalyst and polyolefin product, suspended withinthe fluid medium of the slurry.

As the polymerization reaction proceeds within the reactor 50A, thereaction conditions may be controlled to facilitate the desired degreeof polymerization and reaction speed while keeping the temperature belowthat at which the polymer product would go into solution. As mentioned,due to the exothermic nature of the polymerization reaction, the coolingjackets 52 are provided (around portions of the closed loop system)through which the coolant or cooling fluid is circulated as needed toremove excess heat (heat of reaction) from the reactor, therebymaintaining the reactor temperature within the desired range, generallybetween 150° F. to 250° F. (65° C. to 121° C.), for example. Moreover,it may be desired to maintain the temperature set point, for example,within +/−0.25° F.

In general, reactor temperature varies directly or linearly with changesin the reactor system operating conditions. In certain examples, theheat generated in the reactor by the exothermic polymerization isgenerally linear with the polyolefin production rate (i.e., pounds perhour of polyolefin polymerized). Thus, reactor temperature, which is anindication of the energy or heat in the reactor, may vary generallylinearly with production rate. Therefore, typical reactor temperaturecontrol may involve a proportional-integral-derivative (PID) algorithm.

FIG. 5 is an exemplary cooling system 58 of FIGS. 3 and 4. In thisexample of FIG. 5, the cooling system 58 has a common or shared section66, and individual sections 68A and 68B for the loop reactors 50A and50B, respectively. As discussed, the cooling system 58 providesrespective coolant supply 60A and 60B to the loop reactors 50A and 50B,and receives and cools coolant return 62A and 62B from the loop reactors50A and 50B.

The shared section 66 uniquely provides a common pump 70 that may besized according to the number of reactors supplied with coolant, whichin the illustrated case is two loop reactors 50A and 50B. The employmentof common pump 70, as opposed to multiple pumps dedicated to eachreactor, respectively, beneficially reduces capital and operating costs.The shared section 66 may also include a surge tank 72.

The individual sections 68A and 68B of the cooling system 58beneficially provide separate coolant flow and separate temperaturecontrol for the reactors 50A and 50B. The temperature control for eachreactor 50A and 50B may therefore be improved as compared to a common orcascaded supply of coolant flow to the reactors and a shared temperaturecontrol scheme. Advantageously, the reactor temperature set point of thereactors 50A and 50B may be maintained within +/−0.25° C. in certainexamples with the present techniques. Other tolerances for the reactortemperature set point may be maintained, such as +/−0.2° C., +/−0.3° C.,+/−0.4° C., +/−0.5° C., +/−1.0° C., and so on, including valuestherebetween.

Each individual section 68A and 68B includes a heat exchanger or cooler74A and 74B to remove heat from the coolant. In the illustratedembodiment of FIG. 5, a bypass line 76A and 76B is disposed around eachcooler 74A and 74B. Therefore, as discussed in more detail below, theamount of coolant subjected to the cooler 74A and 74B may be varied butwith the respective coolant flow rate to each reactor beneficiallymaintained relatively constant for hydraulic and heat transferstability.

A variety of coolants may be used to remove or add heat to the reactorsystem. In certain embodiments, steam condensate (demineralized water)is used as the coolant. The coolant return 62A and 62B “carries” theheat removed from the respective reactors 50A and 50B. The coolingsystem 58 transfers this heat to a utility cooling medium (e.g., coolingtower water or sea water) of the coolers 74A and 74B within eachindividual section 68A and 68B. Thus, the cooling system delivers“cooled” coolant supply 60A and 60B to the reactor jackets. Typicalcoolant supply 60A and 60B temperatures may range from 105° F. to 150°F., and typical coolant return 62A and 62B temperatures may range from130° F. to 190° F.

Lastly, respective heaters (not shown) may also be included in theindividual sections 68A and 68B to provide for heating of the coolant,such as during start-up of a loop reactor 50A and 50B. The heaters maybe installed on the various conduits in the individual sections 68A and68B including on a dedicated parallel conduit (see, e.g., FIG. 7). Theheater may be employed, for example, to heat the coolant to facilitateinitiation of the polymerization reaction during start-up of the loopreactors 50A and 50B. The heater may be turned off during normaloperation of the loop reactor.

Referring to FIGS. 6 and 7, two examples of a portion of the coolingsystem 58 of FIGS. 3-5 are shown, respectively. The common section 66services both loop reactors 50A and 50B. The individual section 68A forthe first loop reactor 50A is depicted. The interface between the commonsection 66 and the individual section 68A is indicated by the dottedline 80 (the beginning of the individual section 68A).

For the sake of clarity, the other individual section 68B for the secondloop reactor 50B is not shown. However, in these examples, theindividual section 68B is substantially similar or identical to theindividual section 68A. Therefore, while the discussion with respect toFIGS. 6 and 7 may focus at times on the individual section 68A and thefirst reactor 50A, it should be appreciated that the discussion canequally apply to the individual section 68B and the second reactor 50B.

FIG. 6 is a generally more simplified depiction showing the shared pump70 in the common section 66, and the cooler 74A and bypass line 76A inthe individual section 68A. The temperature of the coolant supply 60A tothe first loop reactor 50A is controlled by modulating the amount ofcoolant flowing through the cooler 74A. In both FIGS. 6 and 7, the flowrate of the coolant supply 60A to the first loop reactor 50A may bemaintained substantially constant by modulating the amount of coolantflowing through the bypass line 76A.

FIG. 7 additionally shows a surge drum 72 in the common section 66, anda heater 84 in the individual section 68A, both of which are discussedin more detail below. In certain embodiments, the temperature of thecoolant supply 60A to the first loop reactor 50A is controlled bymodulating the amount of coolant flowing through the cooler 74A andthrough the heater 84 (whether or not the heater is supplying heat).When the heater 84 is “off” and not providing heat, the heater 84 may bemerely a “wide spot” in what can be characterized as a second bypassline around the cooler 74A. In certain embodiments, this second bypassline (i.e., through the heater 84) may be employed as the primary bypassline (see FIG. 7) for diverting coolant flow from the cooler 74A toadjust coolant temperature, and the first bypass line 76A employed tohelp maintain a substantially constant total flow of coolant supply 60Ato the reactor 50A. Of course, with regard to FIGS. 6 and 7, othervariations of the controllers and control valves are contemplated toprovide temperature and flow control.

Continuing with FIGS. 6 and 7, the common section 66 discharges coolantto the individual section 68A which discharges coolant supply 60A to thereactor jackets 52 of the first reactor 50A. Likewise, the commonsection 66 discharges coolant, as indicated by reference numeral 82, tothe individual section 68B which discharges coolant supply 60B (notshown) to the reactor jackets 52 of the second reactor 50B. In theseexamples, the common section 66 receives the coolant return 62A from thefirst loop reactor 50A, and the coolant return 62B from the second loopreactor 50B. As illustrated, the individual section 68A cools (i.e.,removes heat from) the coolant via cooler 74A (see also FIG. 5), andprovides the coolant supply 60A to the first loop reactor 50A.

The coolant flow through the cooling system 58 and through the reactorjackets 52 of both loop reactors 50A and 50B may be circulated, forexample, by a centrifugal pump, as illustrated by coolant pump 70 (seealso FIG. 5). An exemplary design basis of a coolant pump isapproximately 50 to 60 pounds per square inch (psi) delivered head at 12to 24 million pounds per hour of coolant (i.e., 6 to 12 million poundsper hour of coolant per reactor).

The coolant circulation may be a closed loop, hydraulically full system.Thus, a surge drum 72 (not show in FIG. 6 but discussed below withrespect to FIG. 7) may be employed in the coolant circuit (i.e., at ornear the suction of pump 70) to maintain the circuit liquid full and toreduce swings in pressure of the coolant system by compensating forhydraulic expansion caused by coolant temperature swings. Thus, pressuremay be maintained substantially constant at the pump 70 suction bycontrolling level and pressure of the surge drum 72.

As indicated, the flow rate of coolant through the cooler 74A and 74Band the flow rate of coolant through bypass line 76A and 76B may bevaried to facilitate coolant 60A and 60B temperature control and reactor50A and 50B temperature control. Yet, the circulation flow rate ofcoolant 60A and 60B through the reactor jackets of the reactor 50A and50B is generally maintained substantially constant. Thus, the totalcirculation flow rate of coolant 65 through the coolant system 58 andall reactor jackets is typically maintained constant. However, the totalflow rate of coolant 65 and also the flow rate of coolant 60A and 60B toeach reactor 50A and 50B may be adjusted if desired. Further, it shouldbe noted that the respective flow rate of coolant 60A and 60B to eachreactor 50A and 50B may be the same or different. The coolant 60A and60B flow rate through the jackets for each reactor 50A and 50B may bemeasured via a flow element in the respective individual sections 68Aand 68B, for instance.

In the illustrated embodiments of FIGS. 6 and 7, the circulation flowrate through the reactor jackets of the first loop reactor 50A ismeasured at flow element 96. The flow element 96 may represent, forexample, a flow orifice plate installed in the coolant piping withpressure detection taps disposed on the piping upstream and downstreamof the orifice. A control system distributed control system (DCS) maycalculate the circulation flow rate through the first reactor jacketsbased on the orifice size and the measured upstream and downstreampressures. The flow rate indication from flow element 96 is received byflow controller 98, which may be a control block in the DCS. To maintainsubstantially constant flow through the reactor jackets of the firstloop reactor 50A, the output of flow controller 98, using control signal100, may adjust the position of the valve 102 on the flow bypass line76A (see also FIG. 5). In one embodiment, it is desirable to minimizemovement of valve 102 position to prevent cycling in the coolant pump70. Thus, additional means at other points in the system may assist inmaintaining the total coolant circulation flow rate constant. Moreover,in all, DCS such as that manufactured by Honeywell, Foxboro, Fisher, andso on, may facilitate the control scheme in each individual section 66Aand 66B.

During normal operation of the loop slurry reactor 50A, heat is removedfrom the reactor contents, and heat is exchanged in cooler 74A (see alsoFIG. 5), which may represent one or more coolers. Heat is removed fromthe coolant in cooler 74A to cool the coolant supply 60A to the reactorjackets 52 of the first loop reactor 50A. The cooler 74A may be, forexample, a shell and tube heat exchanger or a plate and frame heatexchanger. A utility cooling medium, such as cooling tower water or seawater, flows through the cooler opposite the coolant, removing heatthrough the heat transfer surface area of the cooler 74A but notcommingling with the coolant. The utility cooling medium flow isrepresented in these examples of FIGS. 6 and 7 by cooling water supply104 and cooling water return 106.

A cooling tower (not shown), for example, may process the circulatingutility cooling medium by removing heat from the cooling water return106 and providing cooled cooling water supply 104. Thus, in thisembodiment, the cooling tower water removes heat from the coolant, whichin turn removes heat from the reactor 50A.

In one example, the cooler 74A represents four plate and frame exchangercoolers that operate in parallel, each cooler having approximately 200stainless steel (SS-304) plates and approximately 1600 square feet ofheat transfer surface, with the heat transfer coefficient varying fromabout 200 to over 800 Btu/hr/sq. ft/° F. as a function of coolant flowrate. In certain embodiments, the heat removed is about 15.5 millionBtu/hr removed per each of the four coolers comprising cooler 74A, andwith a design pressure drop of approximately 3 psi on the coolant side.For the temperature control, coolant controller 108 (coolant temperaturecontroller) maintains the temperature of the coolant 50A supply to thereactor jacket. Coolant controller 108 sends an output signal 110 toadjust the positions of valve 112 (and potentially other valves).

As mentioned, the total coolant circulation may be a closed loop,hydraulically full system. Thus, a surge drum 72 (FIG. 7) may beemployed in the coolant circuit (i.e., at or near the suction of thepump 70) to maintain the circuit liquid full and to reduce swings inpressure of the coolant system by compensating for hydraulic expansioncaused by coolant temperature swings. Thus, pressure may be maintainedsubstantially constant at the pump 70 suction by controlling level andpressure of the surge drum 72.

In the particular example of FIG. 7, a constant pressure gas pad may bemaintained in the overhead of surge drum 72 to maintain a constant pump70 suction pressure. For example, a pressure regulator 88 may addnitrogen to the surge drum 72 overhead and pressure regulator 90 mayremove gas from the overhead. Any suitable gas, such as nitrogen, may beused for the gas pad on the surge drum 72 overhead. The pressureregulators 88 and 90 may be local control valves. A substantiallyconstant liquid level may be maintained in surge drum 72 by adding anddraining coolant via level control valves 92 and 94, respectively.Recovery of the coolant (e.g., steam-condensate, demineralized water,treated water, etc.) removed from the system via level control valve 94may normally not be required.

Again, during normal operation of a polyolefin loop slurry reactor 50Aand 50B, heat is removed from the reactor contents. During start-up ofthe reactor, however, heat is added to the reactor contents tofacilitate initiation of the polymerization. The cooling system 58 maybe used to add heat to the reactor 50A and 50B contents until thepolymerization becomes exothermic.

Therefore, the cooling system may include a heater 84 (FIG. 7), whichmay represent one or more heaters. Steam 114, or some other heatingmedium, flows through steam supply valve 116 to the utility side ofheater 84 to heat the reactor coolant on the process side of heater 84.During startup of reactor 50A, relatively more reactor coolant flowsthrough cooler bypass valve 118, which acts as a heater 84 valve. Thecooler valve 112 is normally closed during startup of reactor 50A andopened after the polymerization reaction becomes exothermic tofacilitate cooling rather than heating.

The heater 84 may be, for example, a shell and tube heat exchanger, amixer, a sparger, or an eductor, such as a pick heater. If a shell andtube heat exchanger is used, the steam 114 is normally not mixed withthe coolant but instead condensed and removed as steam condensate, forexample, via a steam trap (not shown) disposed on discharge 120 from theutility side of the heat exchanger. The condensate may be recovered orsent to drain (sewer). For the case of the heater 84 representing aneductor, such as a pick heater, the steam 114 is mixed with the coolantto heat the coolant via direct steam injection and, thus, the discharge120 is typically not used. In one configuration, a four inch steamsparger adds 300 psig steam directly into the coolant through an 18 inchpiping elbow.

After startup of reactor 50A and when the reaction in reactor 50Abecomes exothermic, the normal operation of the cooling system is toremove heat from reactor 50A. Thus, steam supply valve 116 is closed andno steam 114 is added to heater 84. Coolant will continue to flowthrough cooler bypass valve 118 but is not heated. In certain examples,the valve 118 modulates coolant flow through this primary bypass tomaintain a constant total coolant flow to the reactor jackets, and thusto stabilize flow through the secondary bypass 76A controlled by flowcontroller 98 and flow control valve 102. Other bypass configurationsmay be implemented.

It should be noted that in this example the steam supply valve 116,cooler bypass valve 118, and cooler valve 112 are temperature controlvalves. Indeed, in the example of FIG. 7, the output or control signal110 may from the temperature controller 108 drive the respective valveposition of each of these three valves, to give the desired temperatureof the coolant supply 60A to the first loop reactor 50A, and thusultimately the desired reactor temperature in the reactor 50A. Asindicated, the valve 102 on the bypass 76A is a flow control valve whichmay be configured to facilitate a substantially constant flow rate ofcoolant discharging from the individual section 68A to give asubstantially constant circulation rate of coolant through the reactorjackets of the first loop reactor 50A.

In sum, for certain embodiments during normal operation of theindividual section 68A and during normal production of polyolefin inreactor 50A, steam supply valve 116 is closed. Cooler bypass valve 118and cooler valve 112 are used to manipulate the flow to give the desiredcoolant supply temperature to the reactor jackets. For example, if acooler coolant supply is desired, more coolant will flow through valve112 to give more flow through the cooler 74A, and less coolant will flowthrough valve 118 to balance the total hydraulic flow, maintainingconstant the total coolant flow rate to the reactor jackets 52 of thefirst reactor 50A (FIGS. 2-4). Flow controller 98 and flow control valve102 represent additional means to maintain a constant total coolant flowto the reactor jackets. What is more, it should be noted that theplacement of the coolant system control valves, such as cooler valve112, may vary within the exemplary polyolefin reactor system 20 or otherpolyolefin production systems. For instance, the coolant valve, such ascooler valve 112, may more be disposed upstream of the cooler 74Ainstead of downstream as depicted in FIGS. 6 and 7.

As mentioned, the reactor temperature may generally vary linearly withchanges in the reactor system operating conditions. Typical reactortemperature control may involve a proportional-integral-derivative (PID)algorithm because PID control is generally well-suited for controlling alinear process. An accepted assumption in the art is that heat generatedin the reactor by the exothermic polymerization is linear with thepolyolefin production rate (i.e., pounds per hour of polyolefinpolymerized). Thus, reactor temperature, which is an indication of theenergy or heat in the reactor, varies linearly with production rate.This linear assumption is only an approximation, however, and therefore,the reactor temperature control may also involve a cascade scheme (FIGS.7 and 8). The inner or secondary loop of a cascade control scheme maycorrect for non-linear behavior in the reactor process. It should benoted, however, that the cooling system 58 and associated techniquesdiscussed herein may be applicable to a variety of linear and non-linearheat-generation relationships.

Referring again to FIG. 7 for the temperature control, coolantcontroller 108 (coolant temperature controller) maintains or adjusts thetemperature of the coolant supply discharging from the individualsection 68A to the reactor jackets 52 of the first loop reactor 60A.Coolant controller 108 sends an output signal 110 to adjust thepositions of valves 112 and 118 (and valve 116 during startup). Coolantcontroller 108 receives its set point from reactor controller 122, whichcontrols the temperature of the reactor 50A. The temperature set pointof reactor controller 122 may be entered by the human operator. Thus, inthis example, reactor controller 122 is the primary controller andcoolant controller 108 is the slave controller in a cascade controlscheme.

FIG. 8 is a flow diagram of an exemplary temperature control scheme 130of a reactor 50A or 50B. The controllers referenced in FIG. 8 may be theaforementioned coolant controller 108 and reactor controller 122 (FIG.7). Reactor controller 122 maintains the polymerization temperature inthe reactor 50A or 50B at the desired set point. In the example of FIG.8, the operator inputs (block 132) the desired temperature set point ofthe reactor into the reactor controller 122. This desired polymerizationtemperature may be based on the type of polyolefin or the specific gradeof polyolefin.

The reactor controller 122 reads (block 134) the actual reactortemperature 136 from a temperature indicator or sensor located on thereactor 50A or 50B. This temperature indication (i.e., measurement) maybe accomplished, for example, by a temperature sensor or element, suchas a thermocouple or resistance temperature device (RTD), inserted intoa thermowell that extends into the reactor contents. The reactortemperature 136 (polymerization temperature) measured by thistemperature element may be sent via an electronic signal, for example,to a distributed control system (DCS), a programmable logic (PLC) basedsystem, or some other means of controlling the cooling/coolant system.The reactor controller 122 may be defined within the process controlsystem, such as the DCS or PLC-based system, and configured to read thereactor temperature 136.

The reactor controller 122 compares (block 138) the actual reactortemperature 136 versus the set point entered by the operator. The erroror difference between the set point and measured value drives thecontroller 122 action (output). The tuning parameters of the reactorcontroller 122 decide this action. The reactor controller 122 may writea coolant temperature set point (block 140) to coolant controller 108(FIG. 7) to ultimately help maintain the reactor temperature 136 at itsset point. As mentioned, a typical desired tolerance in a polyolefin(e.g., polyethylene) slurry reactor technology is to control the actualreactor temperature 136 at set point plus or minus 0.25° F. In certainembodiments, the cascade nature of the overall control scheme isreflected by the fact that the reactor controller 122 output suppliesthe coolant temperature set point to the coolant controller 108.

The coolant controller 108 maintains the coolant supply temperature atthe desired coolant temperature set point. The coolant controller 108reads (block 142) the temperature 144 of the reactor coolant supply 60Aor 60B (FIGS. 3-5). The coolant controller 108 compares (block 146) theactual coolant supply temperature 60A or 60B versus the coolant supplytemperature set point supplied by the reactor controller 122. Based onthe difference or error between the measured coolant temperature 144 andthe set point generated by the reactor controller 122, the coolantcontroller 108 may send an output signal 110 to adjust (block 148) thevalve opening positions of the cooler valve 112 and cooler bypass valve118. The coolant controller 108 may also send an output signal 110 toadjust (block 148) the position of steam supply valve 116 (FIG. 7), forexample, during start-up.

In general, as with the reactor controller 122, tuning parameters orconstants define the coolant controller 108 action or output. In sum,coolant controller 108 controls the coolant supply 60A or 60Btemperature 144 needed to maintain the reactor temperature 136 at setpoint of the reactor controller 122, and also works in conjunction withflow controller 98 (FIG. 7) to maintain constant total coolant flow tothe reactor jackets.

Lastly, some configurations of cooling/coolant systems for polyolefinloop reactors may employ a second cooler valve disposed in parallel withcooler valve 112 (FIG. 7). This second valve may not be a spare butinstead sized differently than the first. The two valves may accommodateuncertainty or wide design basis in the reactor temperature control.

FIG. 9 is an exemplary plot 200 of system performance curves. The y-axis202 is valve opening (%), and the x-axis 204 is coolant controller 108output 110 (%). The three curves 206, 208, and 210 represent operatingdata for three control valves in the cooling system 58. Curve 206represents an example of the performance for the cooler valve 108 (FIG.7), which may be the most significant valve in the control scheme incertain embodiments. Curve 208 represents an example of the performanceof the cooler bypass valve 118 (FIG. 7), which is also used as theheater valve on start-up. Curve 210 represents an example of theperformance for steam supply valve 116, which should generally only openduring start-up of the reactor when heat is needed.

In certain examples to generate the system performance curves, steadystate simulations that vary operating conditions, such as productionrate, reactor temperature, reactor slurry density, and temperature ofcooling tower water supply may be run. A percent open value for eachvalve in each simulation run is calculated and plotted versus thecoolant controller output (in percent). The overall objective in thedevelopment of these curves may be to linearize the controller outputwith the reactor polyolefin production rate or the reactor coolant dutyrequirements, and so on. An example basis for setting the controllervalues is: 0% to 34% controller output for maximum cooling and reactorfoul; 35% to 74% controller output for startup and normal polyethyleneproduction; and 75% to 100% controller output for the heating zone andstartup.

In this example, increasing coolant controller output (sent via signal110) correlates linearly with lessening heat generation in the reactor.As controller output 110 increases, it reduces the flow through thecooler valve 108 to increase the reactor coolant supply temperature andalso increases the flow through the cooler bypass valve 118 to balancethe hydraulics to maintain a constant total coolant rate. Furtherincreases in the coolant controller output 110 cross into a region thatrepresents start-up of the reactor, where the coolant system may supplyheat to the reactor. In other words, as indicated the coolant controlleroutput 110 has an inverse relationship with heat generation in thereactor. Therefore, the high end of the coolant controller output inthis example relates to low or no heat generation in the reactor, suchas during startup when heat is supplied to the reactor. Curve 210represents the steam valve opening during start-up, i.e., at the highregion of the x-axis 204 for coolant controller output. In this example,a 100% output on the coolant controller represents the initial phase ofstart-up, requiring the most demand for steam via valve 116 and the mostdemand for flow through the heater 84 (and cooler bypass valve 118). Thesystem performance curves 200 may be input into a DCS calculation block,and used in conjunction with tuning constants of the reactor and coolantcontrollers.

FIG. 10 is two plots 212 and 224 illustrating a cascaded scheme of thereactor controller 122 to the coolant controller 108. The two plots are:an exemplary plot 212 of the reactor controller 122 variables over time(axis 214); and an exemplary plot 224 of the coolant controller 108variables over the same time (axis 214). The upper plot 212 mayrepresent a simulated reactor controller 122 in an existing plant or newplant design. The lower plot 224 may represent a simulated coolantcontroller 108 in an existing plant or new plant design.

In the plot 212 for the reactor controller 122, the process input to thecontroller 122 is the actual reactor temperature 136. The reactortemperature set point 216 is entered by the operator. The reactorcontroller output (control action 218) writes new coolant supplytemperature set points to the coolant controller 108. In thisillustrated example of FIG. 10, the reactor temperature set point 216 inreactor controller 122 is changed from 218.0° F. to 218.5° F. The 0.5°F. is ramped over 2 minutes. The actual reactor temperature 136increases in response to control action 218 increasing the coolanttemperature set point. The reactor temperature 136 increases to matchthe reactor temperature set point. The gain (K1) tuning constant isgenerally tuned to optimize the amplitude of the control action 218and/or reactor temperature 136 response. Any offset 220 (or bias) may bemitigated by adjusting the integral reset (T1) tuning constant, forexample.

As for the plot 224 for the coolant controller 108, the input to thecontroller 108 is the actual temperature 144 of the coolant supply 60A(or coolant 60B if controller 108 is for the second reactor 50B). Thecoolant supply temperature set point 226 is supplied by the reactorcontroller 122 output 218 (from plot 212). The coolant controller 108(output 110) adjusts the valve opening positions of the one or morecontrol valves in the coolant system to control the coolant supplytemperature. In the same example as plot 212, the coolant controller 108receives new set points 226 (corresponding to output 218) from thereactor controller 122. The actual coolant supply temperature 144changes in response to coolant control action 110 of the coolant control108 to match the set point 226 (of the coolant controller for thecoolant supply temperature) received from the reactor controller 122.The ultimate effect of the coolant controller 108 may be to operate incascade scheme with the reactor controller 122 to maintain the reactortemperature 136 at set point 216 (plot 212).

FIG. 11 is a method 300 of controlling reactor temperature in a reactorsystem having multiple polymerization reactors (e.g., liquid-phasereactors, loop reactors, autoclave reactors, etc.) for polyolefinproduction. In producing polyolefin, an olefin is polymerized (block302) in a first reactor to form a first polyolefin, and an olefin ispolymerized (block 304) in a second reactor to form a second polyolefin.The polymerization reactors may be arranged in series or parallel. Asthe polymerization is typically exothermic, coolant is supplied to thereactors (e.g., to jackets of the reactors) to facilitate control ofreactor temperature of the reactors.

In the illustrated method, coolant supply is provided (block 306) via ashared coolant pump to the first reactor through a first cooler and alsothrough a first bypass line disposed operationally in parallel to thefirst cooler. While the total flow rate of the coolant supply to thefirst reactor may be maintained substantially constant, the respectiveflow rates of coolant supply flowing through the first cooler and firstbypass line are generally modulated (block 308) to give a temperature ofthe coolant supply to the first reactor desired for maintaining thefirst reactor temperature.

More coolant supply flow through first cooler and less coolant supplyflow through the first bypass line generally provides more cooling ofthe coolant supply to the first reactor. Such may be performed if thereactor temperature of the first reactor has exceeded set point. On theother hand, less coolant flow through the first cooler and more coolantsupply flow through the first bypass line generally provides lesscooling of the coolant supply to the first reactor. Such may beperformed when the reactor temperature of the first reactor has fallenbelow set point.

Further, coolant supply is provided (block 310) with the same sharedcoolant pump to the second reactor through a second cooler and alsothrough a second bypass line disposed operationally in parallel to thesecond cooler. While the total flow rate of the coolant supply to thesecond reactor may be maintained generally constant, the respective flowrates of coolant supply flowing through the second cooler and secondbypass line are generally modulated (block 312) to give a desiredtemperature of the coolant supply to the second reactor for maintainingthe second reactor temperature.

More coolant supply flow through the second cooler and less coolantsupply flow through the second bypass line generally provides morecooling of the coolant supply to the second reactor. Such may beperformed if the reactor temperature of the second reactor has exceededset point. On the other hand, less coolant flow through the secondcooler and more coolant supply flow through the second bypass linegenerally provides less cooling of the coolant supply to the secondreactor. Such may be performed when the reactor temperature of thesecond reactor has fallen below set point, for example.

An advantage of the illustrated method is that a single common or sharedcoolant pump may be employed in operation to provide coolant supply flowto both reactors, while each reactor (and its coolant supply) has anindependent temperature control. Lastly, it should be noted thatadditional equipment or flow paths may be provided. For example, acommon surge tank may be provided. Further, a respective heater may heatrespective coolant supply to the reactors, such as during start-up ofthe reactors. Also, additional bypass lines, as well as flow controlvalves, may offer additional means of maintaining a consistent flow ofcoolant supply to the reactors. Further, the common or shared pumptypically receives the coolant return from both reactors.

FIG. 12 is an exemplary cooling system 58 having the common section 66and the individual sections 68A and 68B, and that supplies coolant tofirst reactor 50A and second reactor 50B. While it has been stated thatthe foregoing discussion of the individual section 68A and the firstloop reactor 50A may equally apply to the individual section 68B and thesecond loop reactor 50B, FIG. 12 is provided to further emphasize such.

As indicated in FIG. 12, the individual sections 68A and 68B may besimilar or identical. The section 68A has the coolant cooler 74A andflow bypass line 76A. The section 68B has the coolant cooler 74B andflow bypass line 76B. The sections 68A and 68B each have a flowmeasurement orifice 96, control valves 102, 112, 118, and a heater 84.Further, the sections 68A and 68B each have the aforementionedcontrollers 98, 108, and 122, which are not depicted.

It should be noted that the coolant system 58 and its individualsections 68A and 68B, and their control, do not merely only involve amere duplication of parts. The present techniques beneficially anduniquely provide for a common or shared motive force of the coolantcirculation with dedicating cooling and temperature control for therespective reactor. The coolant flow hydraulics are effectively balancedamong the respective reactors and their reactor jackets. For instant,the tuning constants for the controller 98 and flow control valve 102are determined and adjusted to address potential more activity of thisflow control loop in maintaining constant coolant flow to the respectivereactors and for effective operation of the coolant pump 70. Further,the tuning and other factors of the additional controllers in thecooling system 58 are also adjusted as needed.

It should also be noted that the pump 70 may be the only operationalpump in the coolant system 58. In certain embodiments, the pump 70 isthe only pump in the coolant system 58, in that a spare or standby pumpis not provided, as pump reliability may be high in water service.Advantageously, in these cases of no standby or spare pump for pump 70,capital costs may be reduced.

FIGS. 13-16 provide embodiments of the individual section 68A or 68Bwith specific regard to the heater 84 and bypass lines. As stated above,the heater 84 may be disposed on various conduits in these sections.FIG. 13 depicts the section 68A or 68B without a heater 84. In thisexemplary case, heating for the reactor 50A or 50B during startup isprovided by means other than a heater 84.

FIG. 14 depicts the heater 84 disposed on the flow bypass line 76A or76B. A separate bypass or heater line is not employed. During normaloperation with the cooling system 58 removing heat from the reactor 50Aor 50B, no heating medium or steam is supplied to the heater 84.Therefore, during cooling, the heater is “turned off” and coolant merelyflows through the heater 84 without being heated.

FIG. 15 is similar to FIG. 14 except that the heater is disposedupstream of the cooler 74A or 74B and the flow bypass line 76A or 76B.As with the embodiment of FIG. 14, during normal operation with thecooling system 58 removing heat from the reactor 50A or 50B, no heatingmedium or steam is supplied to the heater 84. For the sake ofcomparison, FIG. 16 gives the embodiment presented in FIG. 7.

Lastly, in one example, the total coolant flow discharging from the pump70 for a two-reactor system is about 6000 metric tons per hour (MT/hr).In certain instances, the coolant flow to each reactor may be about 3000MT/hr. In certain embodiments, exemplary production rates of examplepolyolefins (e.g., some polyethylene resins) for the two-reactor systemmay be in the range of about 50 MT/hr to about 90 MT/hr, and with about25 MT/hr to about 45 Mt/hr in each reactor, for example. In examples,the capacity ratio of the first reactor polyolefin production to thesecond reactor polyolefin production may be about 0.5:1 and 1.3:1,depending on turn-down and design considerations, for instance. In atwo-reactor system, exemplary weight ratios of total polyolefinproduction rate (both reactors) to total coolant flow (total dischargingfrom the pump 70 to both reactors) may be about 0.004, 0.005, 0.006,0.007, and 0.008. Similarly, in the two reactor system, exemplary ratiosof polyolefin production rate (per reactor) to coolant flow (perreactor, i.e., combined through a cooler and its bypass or bypasses) maybe about 0.004, 0.005, 0.006, 0.007, and 0.008. Of course, in otherexamples, different values for coolant flow rates, polyolefin productionrates, and the aforementioned ratios may be employed.

IV. Summary

Embodiments of the present techniques provide for a polyolefin reactortemperature control system including a first reactor temperature controlpath for: a first control feed stream (e.g., water) to split into atleast (1) a first cooler zone feed stream to pass through a first coolerzone to produce a first cooler zone output stream and (2) a first coolerzone bypass stream; a first treated stream having a first treated streamtemperature and including the first cooler zone output stream and thefirst cooler zone bypass stream; and a first recycle stream which is thefirst treated stream after the first treated stream has exchanged energywith a first polyolefin reactor. The temperature control system furtherincludes a second reactor temperature control path for: a second controlfeed stream to split into at least (1) a second cooler feed stream topass through a second cooler zone to produce a second cooler zone outputstream and (2) a second cooler zone bypass stream; a second treatedstream having a second treated stream temperature and including thesecond cooler zone output stream and the second cooler zone bypassstream; and a second recycle stream which is the second treated streamafter the second treated stream has exchanged energy with a secondpolyolefin reactor. The first and second cooler zones may each includeone or more coolers (e.g., one, two, four, etc.) such as one or moreplate-and-frame heat exchangers. Lastly, the reactor temperature controlsystem also includes a shared temperature control path configured to:combine the first and second recycle streams to form a combined recyclestream; process the combined recycle stream through shared systemequipment to form a shared output stream; and split the shared outputstream into the first control feed stream and the second control feedstream.

The reactor temperature control system may include a first treatedstream control system configured to control the first treated streamtemperature by manipulating flow rates of one or more of the following:the first cooler zone feed or output streams or the first cooler zonebypass stream. Likewise, the reactor temperature control may include asecond treated stream control system configured to control the secondtreated stream temperature by manipulating flow rates of one or more ofthe following: the second cooler zone feed or output streams or thesecond cooler zone bypass stream.

Additionally, the first control feed stream may be further configured tobe split into a first heater zone feed stream to pass through a firstheater (e.g., shell and tube, sparger, pick heater, etc.) zone toproduce a first heater zone output stream, and wherein the first treatedstream further has the first heater zone output stream. In this case,the first treated stream control system may be configured to control thefirst treated stream temperature by manipulating flow rates of one ormore of the following: the first cooler zone feed or output streams, thefirst cooler zone bypass stream, or the first heater zone feed or outputstreams.

Similarly, the second control feed may be further split into a secondheater zone feed stream configured to pass through a second heater zoneto produce a second heater zone output stream, and wherein the secondtreated stream further has the second heater zone output stream. Asecond treated stream control system may be configured to control thesecond treated stream temperature by manipulating flow rates of one ormore of the following: the second cooler zone feed or output streams,the second cooler zone bypass stream, or the second heater zone feed oroutput streams.

On the other hand, the first cooler zone bypass stream may be configuredto be routed through a first heater zone. In this case, the firstcontrol feed stream may be further configured to be split into a firstsecondary bypass stream through a flow control valve, and wherein thefirst treated stream further has the first secondary bypass stream.Likewise, the second cooler zone bypass stream may be configured to berouted through a second heater zone. In this case, the second controlfeed stream may be further configured to be split into a secondsecondary bypass stream to through a flow control valve, and wherein thesecond treated stream further comprises the second secondary bypassstream.

It should be noted that a heater zone may instead be disposed in otherpositions. For instance, the first control feed stream may be configuredto pass through a first heater zone before being split into at least thefirst cooler zone feed stream and the first cooler zone bypass stream.Similarly, the second control feed stream is further configured to passthrough a second heater zone before being split into at least the secondcooler zone feed stream and the second cooler zone bypass stream.

The shared system equipment may include a coolant pump (e.g., acentrifugal pump) configured to provide both the first control feed andthe second control feed. Indeed, the coolant pump may be configured tosimultaneously provide the first control feed and the second controlfeed. Further, the shared system equipment may include a surge tank.

In operation, the first reactor temperature may be controlled within1.0° F. of a first reactor temperature set point, within 0.5° F. of afirst reactor temperature set point, and/or within 0.25° F. of a firstreactor temperature set point, for example. The first recycle stream andthe first treatment stream have a temperature difference of at least 10°F., at least 25° F., and/or at least 40° F. In certain examples, thefirst reactor may operate with a ratio of reactor production output tofirst treated stream flow rate of greater than 0.004. Similarly, thesecond reactor may operate with a ratio of reactor production output tothe second treated stream flow rate of greater than 0.004.

Additionally, in certain embodiments, the first and second reactors havea combined production output and wherein the shared output stream has ashared output stream flow rate such that a ratio of the combined reactorproduction output to shared output stream flow rate is greater than0.004 pounds polyethylene per pound of coolant (treated water). Lastly,the first reactor and the second reactor may have a first to secondreactor capacity ratio of between 0.5:1 and 1.3:1, for example. In otherwords, the polyolefin production ratio between two reactors may bevaried between 0.5 to 1.3 as theoretical or practical limits in certainexamples. In some instances, the lower 0.5 indicates the turn downcapability and the upper 1.3 may be an upper design limit.

Embodiments of the present techniques may also provide a method ofcontrolling reactor temperature, including: splitting a first controlfeed stream into at least (1) a first cooler zone feed stream through afirst cooler zone to produce a first cooler zone output stream and (2) afirst cooler zone bypass stream; combining the first cooler zone outputstream and the first cooler zone bypass stream to give a first treatedstream having a first treated stream temperature; and recycling a firstreturn stream which is the first treated stream after the first treatedstream has exchanged energy with a first polyolefin reactor. The methodmay further include splitting a second control feed stream into at least(1) a second cooler zone feed stream through a second cooler zone toproduce a second cooler zone output stream and (2) a second cooler zonebypass stream; combining the second cooler zone output stream and thesecond cooler zone bypass stream to give a second treated stream havinga second treated stream temperature; and recycling a second returnstream which is the second treated stream after the second treatedstream has exchanged energy with a second polyolefin reactor. Lastly,the method may include: combining the first and second return streams toform a combined return stream; processing the combined return streamthrough shared system equipment to form a shared output stream; andsplitting the shared output stream into the first control feed and thesecond control feed.

The shared system equipment may include a coolant pump, and wherein theshared output stream is a discharge stream from the coolant pump.Moreover, the method may include adjusting the first treated streamtemperature by manipulating flow rates of one or more of the following:the first cooler zone feed or output streams or the first cooler zonebypass stream. Likewise, the method may include adjusting the secondtreated stream temperature by manipulating flow rates of one or more ofthe following: the second cooler zone feed or output streams or thesecond cooler zone bypass stream.

Further, the method may route the first cooler zone bypass streamthrough a first heater zone prior to combining the first cooler zonebypass stream with the first cooler zone output stream. If so, themethod may include further splitting the first control feed stream intoa first secondary bypass stream through a flow control valve, andcombining the first secondary bypass stream with the first cooler zoneoutput stream and the first cooler zone bypass stream to give the firsttreated stream having the first treated stream temperature. Similarly,the method may route the second cooler zone bypass stream through asecond heater zone prior to combining the second cooler zone bypassstream with the second cooler zone output stream. If so, the method mayfurther split the second control feed stream into a second secondarybypass stream through a flow control valve, and combine the secondsecondary bypass stream with the second cooler zone output stream andthe second cooler zone bypass stream to give the second treated streamhaving the second treated stream temperature.

The method may include further splitting the first control feed streaminto (3) a first heater zone feed stream through a first heater zone toproduce a first heater zone output stream, and combining the firstheater zone output stream with the first cooler zone output stream andthe first cooler zone bypass stream to give the first treated streamhaving the first treated stream temperature. In this case, the methodmay adjust the first treated stream temperature by manipulating flowrates of one or more of the following: the first cooler zone feed oroutput streams, the first cooler zone bypass stream, or the first heaterzone feed or output streams. Similarly, the method may split the secondcontrol feed stream into (3) a second heater zone feed stream through asecond heater zone to produce a second heater zone output stream, andcombine the second heater zone output stream with the second cooler zoneoutput stream and the second cooler zone bypass stream to give thesecond treated stream having the second treated stream temperature.Thus, the method may adjust the second treated stream temperature bymanipulating flow rates of one or more of the following: the secondcooler zone feed or output streams, the second cooler zone bypassstream, or the second heater zone feed or output streams.

Embodiments of the present techniques may provide a polyolefin reactorsystem, having a total reactor system production rate, the reactorsystem including: a first polymerization reactor having a first reactorproduction rate; a second polymerization reactor having a second reactorproduction rate; and a reactor temperature control system having a firstreactor temperature control path, a second reactor temperature controlpath, and a shared temperature control path comprising a pump having asingle pump discharge rate split between the first reactor temperaturecontrol path and the second temperature control path, wherein a ratio ofthe total reactor system production rate to the single pump dischargerate is greater than 0.004, for example. The first temperature controlpath includes a first cooler, and the second temperature control pathincludes a second cooler. Further, the reactor system includes a firstcoolant controller associated with the first temperature control path,and a second coolant controller associated with the second temperaturecontrol path.

Embodiments of the present technique provide for a reactor systemincluding, a first polyolefin reactor, a second polyolefin reactor, anda reactor temperature control system having a coolant pump. The coolantpump may be configured to: provide coolant supply to the firstpolyolefin reactor through a first cooler and a first bypass linedisposed operationally in parallel to the first cooler; provide coolantsupply to the second polyolefin reactor through a second cooler and asecond bypass line disposed operationally in parallel to the secondcooler; and receive coolant return from the first polyolefin reactor andcoolant return from the second polyolefin reactor. Of course, thereactor temperature control system can include the first cooler, thefirst bypass line, the second cooler, and the second bypass line. Thefirst cooler and the second cooler may each be a plurality ofplate-and-frame heat exchangers, for example.

The reactor temperature control system may be configured to maintain asubstantially constant flow rate of coolant supply to the firstpolyolefin reactor, and to maintain a substantially constant flow rateof coolant supply to the second polyolefin reactor. A first flow elementconfigured to measure flow rate of the coolant supply provided via thecoolant pump to the first polyolefin reactor. Likewise, a second flowelement configured to measure flow rate of coolant supply provided viathe coolant pump to the second polyolefin reactor.

The reactor temperature control system may include: a first cooler valveconfigured to modulate coolant supply flow through the first cooler tothe first polyolefin reactor: a first bypass valve configured tomodulate coolant supply flow through the first bypass line to the firstpolyolefin reactor; a second cooler valve configured to modulate coolantsupply flow through the second cooler to the second polyolefin reactor;and a second bypass valve configured to modulate coolant supply flowthrough the second bypass line to the first polyolefin reactor. A firstcoolant controller may be configured to specify percent valve opening ofthe first cooler valve and percent valve opening of the first bypassvalve. A second coolant controller may be configured to specify percentvalve opening of the second cooler valve and percent valve opening ofthe second bypass valve. Further, a first reactor temperature controllermay be configured to provide a set point to the first coolantcontroller. A second reactor temperature controller may be configured toprovide a set point to the second coolant controller. The reactortemperature control system may include a surge vessel to provide surgecapacity in circulation of coolant supply to, and coolant return from,the first and second polyolefin reactors.

Further, the reactor temperature control system may include a firstheater disposed along the first bypass line, and a second heaterdisposed along the second bypass line. If so, a first secondary bypassline may be disposed operationally in parallel with the first bypassline and the first cooler. A second secondary bypass line may bedisposed operationally in parallel with the second bypass line and thesecond cooler. The coolant pump may be configured to provide coolantsupply to the first polyolefin reactor through the first cooler, thefirst bypass line, and the first secondary bypass line, and to providecoolant supply to the second polyolefin reactor through the secondcooler, the second bypass line, and the second secondary bypass line. Afirst flow control valve may be disposed along the first secondarybypass line, and a second flow control valve may be disposed along thesecond secondary bypass line.

Lastly, embodiments of the present techniques may provide a methodincluding: polymerizing olefin in a first reactor to form a firstpolyolefin; polymerizing olefin in a second reactor to form a secondpolyolefin; providing coolant supply via a coolant pump to the firstreactor through a first cooler and a first bypass line disposedoperationally in parallel to the first cooler; providing coolant supplyvia the coolant pump to the second reactor through a second cooler and asecond bypass line disposed operationally in parallel to the firstcooler; and receiving coolant return at the coolant pump from the firstand second reactors. The method may include maintaining substantiallyconstant a flow rate of coolant supply to the first reactor, andmaintaining substantially constant a flow rate of coolant supply to thesecond reactor. Further, the method may include modulating flow rate ofcoolant supply through the first cooler, modulating flow rate of coolantsupply through the first bypass line, modulating flow rate of coolantsupply through the second cooler, and modulating flow rate of coolantsupply through the second bypass line.

The method may include: controlling reactor temperature of the firstreactor, comprising specifying percent valve opening of a first coolervalve modulating coolant supply flow through the first cooler, andspecifying percent valve opening of a first bypass valve modulatingcoolant supply flow through the first bypass line; and controllingreactor temperature of the second reactor, comprising specifying percentvalve opening of a second cooler valve modulating coolant supply flowthrough the second cooler, and specifying percent valve opening of asecond bypass valve modulating coolant supply flow through the secondbypass line. Moreover, the method may include: determining reactortemperature of the first reactor, and designating a set point of a firstcoolant controller that specifies percent valve openings for a firstcooler valve and a first bypass valve; and determining reactortemperature of the second reactor, and designating a set point of asecond coolant controller that specifies percent valve openings of asecond cooler valve and a second bypass valve.

Furthermore, the first bypass line may include a first heater, and thesecond bypass line may include a second heater. Coolant supply isfurther provided via the coolant pump to the first reactor through afirst secondary bypass line, and coolant supply further provided via thecoolant pump to the second reactor through a second secondary bypassline.

What is claimed is:
 1. A polyolefin reactor temperature control systemcomprising: (a) a first reactor temperature control path for: a firstcontrol feed stream to split into at least (1) a first cooler zone feedstream to pass through a first cooler zone to produce a first coolerzone output stream and (2) a first cooler zone bypass stream; a firsttreated stream having a first treated stream temperature and comprisingthe first cooler zone output stream and the first cooler zone bypassstream; a first treated stream control system configured to adjust thefirst treated stream temperature by manipulating flow rates of one orboth of: the first cooler zone output stream and/or the first coolerzone bypass stream; and a first recycle stream comprising the firsttreated stream after the first treated stream has exchanged energy witha first polyolefin reactor; (b) a second reactor temperature controlpath for: a second control feed stream to split into at least (1) asecond cooler feed stream to pass through a second cooler zone toproduce a second cooler zone output stream and (2) a second cooler zonebypass stream; a second treated stream having a second treated streamtemperature and comprising the second cooler zone output stream and thesecond cooler zone bypass stream; a second treated stream control systemconfigured to adjust the second treated stream temperature bymanipulating flow rates of one or both of: the second cooler zone outputstream and/or the second cooler zone bypass stream; and a second recyclestream comprising the second treated stream after the second treatedstream has exchanged energy with a second polyolefin reactor; and (c) ashared temperature control path configured to: combine the first andsecond recycle streams to form a combined recycle stream; process thecombined recycle stream through shared system equipment to form a sharedoutput stream; and split the shared output stream into the first controlfeed stream and the second control feed stream.
 2. The polyolefinreactor temperature control system of claim 1, wherein the shared systemequipment comprises a utility cooling medium.
 3. The polyolefin reactortemperature control system of claim 2, wherein the utility coolingmedium does not include a cooling tower.
 4. The polyolefin reactortemperature control system of claim 1, wherein the second polyolefinreactor comprises at least one polymer discharge having from about65,000 to about 200,000 pounds of polymer discharge per hour.
 5. Thepolyolefin reactor temperature control system of claim 1, wherein the atleast one polymer discharge on the second polyolefin reactor is chosenfrom at last one of a continuous take-off (CTO) valve, a settling leg ora pulsating valve.
 6. The polyolefin reactor temperature control systemof claim 1, wherein the first polyolefin reactor comprises at least onepolymer discharge.
 7. The polyolefin reactor temperature control systemof claim 6, wherein the at least one polymer discharge on the firstpolyolefin reactor is chosen from at last one of a continuous take-off(CTO) valve, a settling leg or a pulsating valve.
 8. The polyolefinreactor temperature control system of claim 1, wherein the ratio of thecapacity of the first polyolefin reactor to the capacity of the secondpolyolefin reactor is from about 0.5:1 to about 1.3:1.
 9. The polyolefinreactor temperature control system of claim 1, wherein the flow rate ofthe first control feed stream and the second control feed stream remainsubstantially constant.
 10. The polyolefin reactor temperature controlsystem of claim 1, wherein the first cooler zone comprises at least oneplate-and-frame heat exchanger, and wherein the second cooler zonecomprises at least one plate-and-frame heat exchanger.
 11. Thepolyolefin reactor temperature control system of claim 1, wherein thefirst cooler zone is configured to receive cooling water to lower atemperature of the first cooler zone feed stream through the firstcooler zone, and the second cooler zone is configured to receive coolingwater to lower a temperature of the second cooler zone feed streamflowing through the second cooler zone.
 12. The polyolefin reactortemperature control system of claim 11, wherein the first cooler zonefeed stream and the second cooler zone feed stream supply comprisestreated water, and the cooling water comprises cooling tower water. 13.The polyolefin reactor temperature control system of claim 1, whereinthe combined flow rate of the first cooler zone feed stream, the firstcooler zone bypass stream, the second cooler zone feed stream, and thesecond cooler zone bypass stream is from about 12 to 24 million poundsper hour.
 14. The polyolefin reactor temperature control system of claim1, wherein the weight ratio of total polyolefin production in the firstpolyolefin reactor to the flow rate of the first treated stream and theweight ratio of the total polyolefin production in the second polyolefinreactor to the second treated stream is greater than 0.004.
 15. Apolyolefin reactor temperature control system comprising: (a) a firstreactor temperature control path for: a first control feed stream tosplit into at least (1) a first cooler zone feed stream to pass througha first cooler zone to produce a first cooler zone output stream and (2)a first cooler zone bypass stream; a first treated stream having a firsttreated stream temperature and comprising the first cooler zone outputstream and the first cooler zone bypass stream; a first treated streamcontrol system configured to adjust the first treated stream temperatureby manipulating flow rates of one or both of: the first cooler zoneoutput stream and/or the first cooler zone bypass stream; and a firstrecycle stream comprising the first treated stream after the firsttreated stream has exchanged energy with a first polyolefin reactor; (b)a second reactor temperature control path for: a second control feedstream to split into at least (1) a second cooler feed stream to passthrough a second cooler zone to produce a second cooler zone outputstream and (2) a second cooler zone bypass stream; a second treatedstream having a second treated stream temperature and comprising thesecond cooler zone output stream and the second cooler zone bypassstream; a second treated stream control system configured to adjust thesecond treated stream temperature by manipulating flow rates of one orboth of: the second cooler zone output stream and/or the second coolerzone bypass stream; and a second recycle stream comprising the secondtreated stream after the second treated stream has exchanged energy witha second polyolefin reactor; and (c) a shared temperature control pathconfigured to: combine the first and second recycle streams to form acombined recycle stream; process the combined recycle stream throughshared system equipment to form a shared output stream; and split theshared output stream into the first control feed stream and the secondcontrol feed stream, wherein the first cooler zone feed stream, firstcooler zone bypass stream, second cooler zone feed stream, second coolerzone bypass stream, first treated stream, second treated stream, andcombined return stream are connected in a closed loop, hydraulicallyfull system.
 16. The polyolefin reactor temperature control system ofclaim 15, wherein the shared system equipment comprises a utilitycooling medium.
 17. The polyolefin reactor temperature control system ofclaim 16, wherein the utility cooling medium does not include a coolingtower.
 18. The polyolefin reactor temperature control system of claim15, wherein the ratio of the capacity of the first polyolefin reactor tothe capacity of the second polyolefin reactor is from about 0.5:1 toabout 1.3:1.
 19. The polyolefin reactor temperature control system ofclaim 15, wherein the first cooler zone comprises at least oneplate-and-frame heat exchanger, and wherein the second cooler zonecomprises at least one plate-and-frame heat exchanger.
 20. Thepolyolefin reactor temperature control system of claim 15, wherein theweight ratio of total polyolefin production in the first polyolefinreactor to the flow rate of the first treated stream and the weightratio of the total polyolefin production in the second polyolefinreactor to the second treated stream is greater than 0.004.